It isn't morally wrong as infatuation cannot be helped and typically does not last long (infatuation is the earliest phase of romantic relationships and usually lasts up to two years maximum).
What can be an issue is the imbalance of power.
A younger partner can sometimes be drawn to an older partner as an emotional/intellectual crutch. Their relationship may prevent them from embarking on typical developmental milestones (gaining independence/autonomy). If the relationship is healthy and respectful it is not an issue but the intentions of many men hankering for younger women are purely sexual and often on some level about power/control.

according to the original "wimmens libbers" [small f feminists] the aim was to "empower" themselves using sex as the bait, and it worked.
so we see the "free love" Lolita character of 1960s morphed into the identical [Hayes/Haze] character Angela 30 years later BUT so preoccupied with using sex as a method of advancement that she is just a frigid virgin, over the hill at just 18.
Lester was her only chance but fag kills him.
http://www.kindleflippages.com/ablog/

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It's not morally wrong. But when you act on it, pursuing the object of your affection, that's when you cross the line into immorality.

The sun is shining but the ice is slippery.

I suppose it's not immoral if the love is truly mutual, but it's very frowned upon. The generally held belief is an underage teenage girl is too emotionally immature compared to a grown man and he's most likely manipulating her or using her.

We
have social agreements as to behaviour
And that ranges from sensible to stupid depending on the WE.
Remember Mendes is showing his version of Lolita Haze [as Angela Hayes] just 2 years after the 1997 version of Lolita proved exactly how stupid "morals" have become in america.
http://www.kindleflippages.com/ablog/

No. Nothing you
feel
is morally wrong. We have social agreements as to behaviour, which is the foundation of morality. That's another matter.
"What can be asserted without evidence can be dismissed without evidence."

You need to decide this on your own. I have friends that have a 20-30 year gap and they are married and have kids (She was 23 he was 47). There are going to be eyebrows raised when this happens but if both are adults and consenting you just roll with the punches. In their case it was super dicey since I think the age of consent is 21 in the States (or some states?) so probably best to leave that in the "fantasy land" area.

Nice thing about "morals" is that "immoral behavior" usually only "offends the values" of the person making the allegation and no real harm is done to anyone unless the person affected [Angela] is SWAYED by such morals and becomes a maladjusted person sexually, ie over the hill at 17 [as in Jean Brodie].
But cunning thing is Mendes shows behaviour of the same Angela in control of a car while smoking pot, a practice that can cause death to various innocent people [including Angela].
BUT because that does not involve sexual matters it is not Carefully Taught in schools as being immoral.
It's just the Beauty.
http://www.kindleflippages.com/ablog/

Because there weren't any 18-year-old women hanging around his house.
Choocheechoo choocheechoo choocheechoo ya ya pow!

I would ask why would man go after jailbait. If he can find a 18 year old instead.

when you're my age
May we ask what age that is?
http://www.kindleflippages.com/ablog/

No, especially if you don't do anything about it. That ship has long sailed when you're my age, and I'm off on the sailing ship while the 16-year-old girls are back on the distant shore, golden brown in the sunset.
I'm not going to date young girls at my age, but I'm not going to go around pretending I don't still have any lingering attraction to them. I haven't been hit in the face with a shovel-full of dirt yet.
"Let be be finale of seem/ The only emperor is the Emperor of Ice Cream"

imbalance of power
So just WHO "was doing the chasing" as Night puts it - who are you saying had the power in the Lester/Angela bit? [or 50 years before for Humbert/Lolita]
http://www.kindleflippages.com/ablog/

It depends how you would define "morally".
If there is no harm being done to either party, it's OK I suppose, but there will be an imbalance of power, so maybe that's what makes it morally wrong, that they aren't on the same footing.

Case in point, my father was 25 when he married my mother at 16. They stayed together and had a family. I am proof of that!
On the overall issue, I just watched "The Last of Robin Hood" with Kevin Klein playing Errol Flynn, the famous adventure actor of the 1930s, 1940s, and 1950s. Flynn had a penchant for younger females and this movie covers the last two years of his life when he started his last relationship at age 48 with a girl of 15.
.... TxMike ....
Take a risk, Take a chance, Make a change.
Kelly Clarkson - Breakaway

Part of the problem is American morality vs global morality, which is different with different cultures.
In ancient times, in the Middle East, Jewish and Arab cultures would marry young daughters as young as 14, and to men of varying ages.
I am personally uncomfortable with the idea of a man with a girl under 18, and am certainly more ok with a girl over 25. It is, however, legal in some states to marry a girl as young as 16.
After thinking it through, it is ok but a dangerous area to be in. Much safer, regardless of the age gap, to go with over 18.
Unlike some people. I don't think that a wide age gap is a big deal, just as long as everyone is over 18.
To be
infatuated with a much younger girl
is perhaps technically not a moral issue, because the only real infraction would occur if acted upon (in the case of Christian spiritual belief, of course, even the thought of having sex with her is a sin).
"If you love Jesus Christ and are 100% proud of it copy this and make it your signature!"

Why is it morally wrong to be infatuated with somebody else if you are married or in a serious relationship. People cannot control their feelings.
Yes indeed and the question is if one acts on them or REVEALS them.
That was the issue in Eyes Wide Shut [from Dream Novel] ie the wife actually revealing her "Eve" desires [aka Eve Syndrome].
Lean's Ryan's Daughter perhaps is the best depiction of just how powerful these feelings can be for a woman, enough to send Pauline Kael into a feminist fury
http://www.kindleflippages.com/ablog/

Why is it morally wrong to be infatuated with somebody else if you are married or in a serious relationship. People cannot control their feelings. There is a difference between having those temporary feelings and choosing to act on them. Simply having them is natural, even if you are with the love of your life.
Sean Bean's must see roles: Accused, TLOTR, Ronin, Clarissa, Essex Boys, GOT, Sharpe, Black Death

Depends on a number of factors. And your definition of "infatuation".
Let's use this one,
" an intense but short-lived passion or admiration for someone or something. "
If the 37-yr-old man is married or in a serious relationship already, then becomes infatuated with a 16-yr-old then by most definitions of "morality" that is wrong. But since it is very temporary it is only a very minor morality shortcoming.
If the 37-yr-old man is entirely "single" and no laws are broken then it is NOT morally wrong. Single guys do this all the time!!
.... TxMike ....
Take a risk, Take a chance, Make a change.
Kelly Clarkson - Breakaway