I think you should be very proud of yourself. At only 18 years of age you are asking questions of yourself that many adults three times you age can barely come to grips with. My advice would be to take things a little slower. Every decision has three answers: yes, no, and I need more information. It sounds like you are in this third category when it comes to your sexuality. There might be a multitude of reasons why you may not be attracted to a certain partner’s genitals, but in the grand scheme, the only thing that matters is the relationship you have with yourself. I would advise you to only go as far sexually as you are comfortable, and to listen to what your body is telling you. If you are having fun and you’re attracted to someone and want to be intimate, great. But if you’re not into it, don’t feel that you need to be what somebody else wants in order to please them…
With that said, there are no rules to what a trans person needs to do in order to be themself. Not all transmen undergo phalloplasty, just as not all transwomen undergo vaginoplasy. And not all transmen wear men’s clothing 100% of the time, and not all transwomen wear women’s clothing 100% of the time… What’s important is the alignment you feel within yourself, and if you feel that your body and mind are in harmony the way they are, there is no need to fulfill any ‘rules’ you might hear about.
Gender and sexuality can fit together in any combination you like. Finding out which one (or ones) is/are for you is part of the fun. Don’t rush, enjoy the journey and always remember that you’re not alone.
All the best,