Right now, Mystique is basically "straight with an exception." There's nothing on-panel that actually confirms any sexual or romantic interest in women other than Destiny, while there's extensive evidence of romantic and sexual interest in men. And even her relationship with Destiny was largely subtext (partly because Destiny died before the ban on gay characters was lifted).
I'm not attracted to men. If I wind up falling in love with a guy, I'm still going to identify as straight, because I still wouldn't be attracted to men. That's the situation, right now, with Mystique, at least until some writer actually decides to put something on-panel that does confirm that, yes, she is, in fact, bisexual. She is,
at the absolute best, a closeted bisexual.
And no, a woman doesn't stop being bisexual just because she sleeps with men. But when she
exclusively sleeps with men, flirts exclusively with men, and otherwise shows an interest exclusively in men, then she definitely leans a lot more towards the straight end of the spectrum.