thats sexist

As others have said, it's probably due more to the fact that outside of particular subcultures (like goth, industrial, etc) or Hollywood men don't really wear a lot of different options for clothing compared to women rather than blatant sexism. At least here in the United States (where Sims is based.) I can't say what the situation is like in other countries. But at least in my part of the US, I've never really seen guys in anything other than the basics we already have for them in CAS. (T-shirts, jeans, shorts, sneakers, combat or cowboy boots, jackets, flannel, button down shirts, hoodies, muscle shirts.)

I do wish there were more options for male Sims, and agree that there could be more effort put into giving us more male clothes, but the reality is, it is very rare to see guys in traditionally more "feminine" (as in clothing aimed at women) clothing like mesh tops for example. Yes, there are men who will wear them, or whatever else they want regardless of what society says is "feminine" or "masculine," but I'm just saying that it's very rare.