Why would a male to female transgender only seek relationships with male to female transgenders?

Not all MTF want to be with other MTF. One stereotype is that all transgender people are homosexual. However, this is absolutely not true. Being trans is not in itself a sexual orientation. You can trans and be homosexual, but you could also be straight. You could like boys, girls, both, other trans people. It doesn't really matter. It's just like any other person, only in the process one wishes to actually be the opposite sex.