On top of everything how could he be the bio dad of that beautiful boy, impossible simply impossible!

Why did they have to go with the stereotype of the effeminate Beta Jewish male

How is the one kid who is so obviously a gay male the only one that the writers haven't made into a homosexual?
Oh, I can buy Allie playing with lesbianism as a "hip" thing to do, or to seek attention. But the older daughter throwing away her children and everything for a pushy dick of an ex-girlfriend who she (possible spoiler follows) gets tired of and casts off in season two is not really working.

I didn't buy Ali as promiscuous. She is very unattractive, It's impossible to picture anyone having sexual interest in her.
That said, i guess her hooking up with lots of bottom-of-the-barrel-in-looks men is plausible

Women with low standards who fall for the insecure hipster routine there are a few of them, I'm not surprised.
I don't generally care about looks but he wouldn't get within 20 feet of me without shaving that ridiculously shaggy beard. He looks like a homeless guy and it's very definitely creeping me out.
For every lie I unlearn I learn something new - Ani Difranco

I also didn't buy it but then I thought of John Mayer and how he doesn't look nothing like a ladies man but he ended up hooking up with a lot of hot chicks.

He's definitely a NO.

One of the best, articulate posts I've seen. Thank you.

Great reply! Very well put.

I think it's more complicated than that.
Looks-wise, I've only seen Jay Duplass on The Mindy Project before this, where I thought he was pretty slubby. Maybe it's the weight loss and haircut, maybe it's the acting, but on Transparent I thought he was a not unattractive, regular-looking guy. Given that he at least has the appearance of financial stability (a career he loves and his own home), there is nothing on the surface that screams 'ewww'.
Moreover, the women Josh was hooking up with were more interesting than beautiful. So, they didn't really come off as just about looks. Carrie Brownstein's character Syd made an interesting point about him having a neurotic intensity that kind of demands/begs for a woman to love him, with an implicit promise that if that happens he'll love her in return. Alli (Gaby Hoffman) was probably right about him being a love addict, hence why Raquel (Kathryn Hahn) ran off when she realized that might be true. Peopleespecially ones with other issuescrave love, so that probably adds to the attraction. Only with love addicts, they lack the rationality to see when those feelings are temporary because it's really just lust.
The one thing he has in common with Jason Stackhouse is that they both had early sexual experiences with older women in authority positions. Josh might be more similar to Dawson's Creek's Pacey Whitterwho also hooked up with an older woman in authority, but was more of a "boy least likely"-type. In which case, a certain confidence in being able to connect with a woman in a way that her peers can't can obfuscate his other flaws and insecurities. A big part of that is being able to tell the women what they want or need to hear to feel good about themselves, and him believing itat least at the moment.
Josh misreads the musician girl, but I think that was more of a device to show how his neurosis plays into these relationships. He lives in the fantasy of the moment, not reality. Hence whydespite projecting a desire for potential children with the musician girl and Raquelhe is flummoxed by the reality of a grown child with the supposed "love of his life."

Not really a lady's man, more of an insecure sex addict I think.

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He has an insanely nice house and is a record exec of some sort. He could be 300 pounds and he'd still be swimming in strange.
Pretty clear from the show, anyway, that he only tends to sleep with musical acts he manages. And all of those women are clearly using him, given that none want to commit.
I don't think he's supposed to be a ladies' man as much as a kind of shallow loser who's gullible enough to think that women are sleeping with him because he's a great person, as opposed to because they want to get ahead.