Grab-n-Stash said:
One thing that always puzzled me is how I've never seen a version of those hoodies that have "He's mine" and "I'm his" or vice versa

I can think of two reasons: first is that homosexual couples are, necessarily, a subset of a subset of a subset _{(homosexual -> homosexual couples -> m/m OR f/f couples)} so marketing to them isn't quite as lucrative. second, there is the potential that people will see the "She's mine" and "I'm his" shirts as kind of misogynistic.