Your demeanor and personality sound like what some people (erroneously) describe as "straight-acting." The thing is, you've made it very clear that you're not straight, and you're not acting: You are living life as your authentic gay self. No one, be them inside or outside the queer/LGBT community, has the right to tell you not be who you are or should be, and the people in the LGBT group who criticized you for "living in the binary" and "not being who you truly are" were absolutely in the wrong to do so. And no, you don't have to wear skirts around campus if you don't want to. No one is going to take away your gay card because you only wear garments with legs.
Which leads well into the subject of drag. Personally, I think it was awesome that you devoted time and energy to help with setting up the drag show and working on the lineup for it, even though drag isn't an interest of yours. That you felt pressured by your peers to perform in the show was again unfair. There are quite a lot of gay men who don't have any problems "fully expressing who we are" as gay men without doing drag, and the person who told you otherwise was simply wrong. There are a host of reasons someone might not want to perform in a drag show, but "I don't want to" should be all the reason someone needs to give. Not that I personally don't enjoy a spot of drag now and again, but I'm not you.
Unfortunately, the use of "homosexual" is going to have to be one of the places where I'm going to disagree with you. You'll find as you come into contact with the broader LGBT demographic that a lot of people, possibly even a majority, find "homosexual" to be a somewhat offensive word. Feel free to blame the religious right for ruining "homosexual," regardless of what you see as its technical accuracy. The clinical and dehumanizing way it has been used by the right in their rhetoric over the years has pretty much destroyed most gay people's ability to have any positive associations with it.
Moving along, I promise that you and your boyfriend are far from the only LGBT engineering or science majors out there! It sucks that your choice of major was another way in which you felt you didn't fit in with the other students in the LGBT student group. The fact that the group's career-oriented programming was exclusively aimed at the art and social science students does a real disservice to other students at the university. Some part of why the group had such a strong art and social science slant may well be because those are the students more likely to have come out younger, and hence be comfortable being involved in a university's LGBT student group. Which isn't to say that it wouldn't be good for the group to have done more outreach to other majors.
Now that I have covered some of your concerns, I'd like to give you some perspective and context for why you experienced some of what you did.
Try to understand that, for a lot of the students in the LGBT student group, being part of that organization may have been the very first time they felt a sense of belonging, or that they could be free to be themselves. It's safe to say that a lot of queer/LGBT students did not fit in too well in high school, and if they didn't actively catch a good bit of hell for being who they were, it was likely a very real fear.
And here's the thing: the people who gave them that hell... probably looked and sounded an awful lot like you.
That's not your fault, and you shouldn't feel in any way as if you need to shape your behavior to fit some gay archetype that does not line up with who you are as a person. It is unfair that you didn't experience that same welcoming and acceptance, perhaps because you may have reminded some of your fellow LGBT students of the people they feared, or who made their lives hell when they were younger. That isn't really their fault either though: We are all the product of our experiences.
Hopefully considering that background can help you understand why you may have experienced some of the things that you did. When other students in the LGBT group were trying to convince you to change your mannerisms, clothing, gender presentation, or to do drag, what they were really saying was "please show us that you're different than you seem, that you're not like [insert tormentor name here], who never would have been willing to do [X] thing that violates societal norms."
The good news is that it isn't going to stay like this forever. As everyone gets older, more and more people like you - that is to say LGBT people who are more commonly read as straight/cis by society - will get comfortable with themselves and seek out LGBT community. At the same time, the sorts of people you encountered in the LGBT student group will meet many more queer/LGBT people who don't fit their particular vision of how to be queer/LGBT, forcing them to expand their concept of what it means to be part of this community and become more accepting of people who are different than themselves. Hopefully along the way the media will continue to improve its portrayal of LGBT people, and Western culture will further embrace the idea that LGBT people are as diverse as any other population.
Winter (on behalf of the generation before you)
P.S. All that said, try lounging about the house in a skirt sometime. It's like wearing boxers only better