Generally when women become emotionally distant, that's when they're doing it.

Sometimes, but not necessarily.

The premise behind your thinking is that women are as attracted to men as men are to women and that women are as interested in sex, intimacy and sexual intimacy with men as men are with women. Therefore you assume that if she isn't getting it from you, she's getting it from someone else.

None of that is true. The female desire for sex, intimacy and men in general, is much lower than our desire for them.