There are many jokes to be made here, but the truth is: it doesn't make you gay

Guys think that sexual pleasure comes 100% from anterior part of the genitals, the penis and scrotum, but they are missing out on the pleasure provided by the nerve endings found in and around the distal part of the anus, and not to mention the direct stimulation of the prostate, that can lead to a very powerful orgasm. Why do you think some women and gay men enjoy this: because we are built to enjoy ourselves this way as well

Also anal penetration is a form of submission, so some people with dominant personalities, prefer this sexual act as a form of release

Now, on the otherhand if you are fantasing about a man while a women does this to you, and have sexual fantasies about men, then maybe you are gay or bi