If you're a man who's attracted to someone packing a peen, be it under a snappy suit or a dress, then you're not straight. You're gay or bi, and possibly in deep, deep denial. But you're not straight.
Female-to-trans are more often lesbian than they are straight. So they end up in relationships with lesbians. But those are lesbian relationships, XX to XX, not straight relationships. It's not unheard of for FtT to want to have relationships with gay men (most of whom are not enthused about this, but don't come in for the same stick as penis-refusing lesbians). When this happens, it's a straight relationship, XX to XY, so the men involved are again at least bisexual. Male-to-trans are sometimes gay, so want relationships with gay men - XY to XY. But frequently they are straight, so want to have relationships with women, most of all lesbians. XY to XX = straight relationship.
It's all very simple if you set aside who identifies as what and think about who could technically impregnate whom. The rest is just sexual preference, which, for clarity, matters not a jot to me - sleep with who you like! But don't give other people shit if they don't fancy you - that's just life.