It isn't even about bigotry or homophobia... it's possible to be uncomfortable with the idea of being in a relationship with someone who used to be, at least physically, the opposite gender from the one you are interested in without being outright homophobic.
It's about having realistic expectations of people's reactions to things.
If someone is going to be fine with you being trans, they are going to be fine with it. It's a very personal thing, far beyond simply accepting the existence and their right to live their lives when it directly affects you in such an intimate manner.
Sexuality is an incredibly personal and emotive thing for people. People can argue all they want that it shouldn't affect people, but the fact of the matter is that it WILL... and it could affect them in some highly negative ways that will stay with them for life.
If you suggest that you care about a person enough to be in a relationship with them, frankly it doesn't speak well for that if you keep something so massive and potentially affecting from them. Worse, even if they might be willing to accept it, you'd pretty much have to be a trans person who knows they will 100% not risk it coming out in the future due to attempts for children, or even social interaction with family or friends from earlier in your life, and probably causing major issues in the relationship then.