Xavin . . . X-Men. This one is a little bit difficult to call, similar to Mystique. Xavin is a skrull, which is a race of alien beings that can shape shift. Xavin spends time in male and female form, however she is in a relationship with the lesbian Karolina Dean. At one point, Xavin, in skrull form, was very upset and transformed into a human female, which Dean thought that she is female at heart.
Mystique was probably one of the first lesbians in the Marvel Universe. Although her sexuality in recent years can be described as bi-sexual (and it's even unclear if she is a woman, man, transsexual, or something else entirely), she was originally written to be in a relationship with Destiny, but at the time, Marvel would not allow the writers to be that explicit.