Yeah, she doesn't love him - in fact, she hates him. (sexual abuse the first time they met, and all that) And he likewise, should logically hate her. (double-crossed and ended up quite brutalized because of her, again, the first time they met.)
Yet, they've actually hooked up through the years... sometimes with decades in-between, but none-the-less. They even had a kid together! Which they sent away for adoption immediately, but none the less, a kid...
They both have psychopathic disorders to one degree or another, but we do know that Mystique CAN love someone - she did love Destiny, but arguably she might actually be the ONLY one she's ever loved, perhaps not even her children.
Sabretooth...? Ehh, well, I don't know... MAYbe he is capable of love... it's hard to find any evidence of such, tho'.
But... why do they keep getting together, in a way? Is this what is known as a hate-love relationship, or something? I.e, a bit like Daredevil and Typhoid Mary.