That sucks.
I don't know what gender or orientation your friends are. But what they are doing sounds like what a lot of gay people get. How do you know you are a gay man if you haven't slept with a woman/guy before?
It often helps to turn it back on the people questioning. If someone straight says, "You're not gay, you just haven't met the right man/woman," you should ask them if they've ever had sex with the type they are not attracted to, and if not, then how do they know they are not gay/straight.
Here, though, it sounds like they are just very confused about orientation and identity. Sleeping with a girl will not prove you are trans, because lesbians also sleep with girls. So the argument doesn't even make sense that you're 'only' a lesbian and not trans, due to not having sex with women yet. And telling you that you're not a REAL dude unless you've slept with a woman? That sounds kinda like...they are treating you like a guy. That's normal stuff for one guy to tell another and goes to conceptions of manhood.
The other thing I would want to chime in on is this idea of making "final decisions." If people are talking about your sexual orientation, it's really only their need to label you that requires 'final decisions.' You are NOT making any permanent choices about who you will be attracted to or sleeping with in the future, and there is no reason to view it as such or require labels that make it so. They can't tell you that you do or do not like girls *that way*. So it really is about them trying to find the right label for person like you who likes girls...
...which leads us to your being trans. People who do not understand that you ARE a guy, what gender identity is, and how transitioning is about making your outward expression and appearance more congruent with your insides, will get stuck on the labels of orientation as they connect to your gender. This is where the "final decisions" talk gets very annoying. It's very common for people to ask us why we need to transition or why don't we wait or 'hold off making decisions' about being transition, since it is life-altering and includes some permanent effects (if you go medical route). It sucks playing trans 101 educator, but while people commonly now know what gay is (and they didn't 40-50 years ago!), they do not realize that the steps of transitioning are not the same as making "final decisions" to CHANGE your sex/gender. You already ARE male, and transitioning is just about what steps make you feel more comfortable expresssing that or being seen that way. Sometimes it helps to ask people: if you truly believed I was a guy, would you say this/act this way/ask me to do this? Or you can say, "Do you actually believe I am male?" THe answers - which you may not like - tell you a lot about how to respond or whether these people can/will be friends in the future. It's not about whether they agree with transition, or can adjust to new names/pronouns. It's about whether they are willing to understand what gender identity is and believe that you are who you say you are.