Why do gay guys act like women?

For the most part, they don't. Most of whom you believe act as women represent a fraction of all gay men.

Of the more effeminate gay men, most don't really act that much like women. If they do, it is usually only confined to partner seeking behaviors. Even of the ones who seem more "feminine" in general, there are still differences in their mannerisms which are often more ostentatious than with women. Their "feminine" expression is often done with masculine motives and style. So saying gay men act feminine is usually an over-simplification, and is often based on outdated and sexist views. Most gay relationships are "equality-based," meaning there is not one filling a "guy" role and one filling a "girl" role.

Now keep in mind that a good portion of what you think are effeminate gay men might actually be transgendered persons (the ones who dress or live as the other sex, but usually never go as far as get surgery). Transgendered persons can be of any sexual orientation, or even not be interested in sex at all. Roughly a third of all male-born transgendered persons are exclusively attracted to persons opposite their born genital sex. They may consider themselves lesbians. So men who live as women are often attracted to women, thus meaning they are not gay guys.