They can feel emotions. The voice over clearly states that it is just the highs and lows of emotion that the drug limits. Consider it being on strong xanax in addition to the society encouraging self control at the top of that.

I think you're projecting
Good for you

In a man's case, yes.
Can't speak for the women though
All these voices in my head, and not a single one I understand.

When you decide to get married it's usually for love?
Are you for real?

Preston had a wife, and, when you decide to get married, it's usually based on love.
That is a big fat no. Marriage is institution, economical contract, love is quite a recent addition. Marriage brings structure into chaotic life.

Preston had a wife, and, when you decide to get married, it's usually based on love.
Usually. Not always. And throughout history, arranged marriages had nothing to do with love. Heck, even in modern times, there are countless loveless marriages.
and Preston masturbated in a cup (which would require being horny as well),
No. It wouldn't. The body can still react to physical stimulation despite not being "in the mood."
Seize the moment, 'cause tomorrow you might be dead.

The whole point of marriage in the movie was to showcase "STRUCTURE." Which is important for controlling the populace. Without assigned partners and 2 children per couple there would be more chaos and lack of organization.
A bright light can hollow the deepest of nothingness.