I get why you are asking this question. I am a female and was with some one for 4 years she was always between 105 and 110. Going into our fourth year she started to gain weight. She had thicker thighs and booty and was now between 115-120.
After that year passed she told me she was cheating on me with a man that whole year. They were having intercourse and she was trying to get pregnant . I don't think her weight gain was a coincident. Especially since once her intercourse with this man stopped her weight went back to normal.
What do u guys think?

I have moved your post to its own thread so that you can get answers for your question.

How old was your ex? Other than cheating on you did she have any other lifestyle changes such as not exercising, diet, etc.

Stress from cheating could have been a reason for the weight gain as much as the sex.

A person's weight fluctuates constantly; plus to minus 5-10lbs isn't unusual. Chances are it wasn't the sex but the other associated activities that occur during courting that caused any weight gain. IE, eating together or post coital snacks or what not.

I can honestly think of no reason why sex in upon itself would cause weight gain.

The cheating... well that is a completely different issue. Are you still with her? If so, why?

I am totally conflustered by your question. You were cheated on for a whole year by someone trying to get pregnant and you ask about weight gain?

Hopefully you have moved on.