Honestly, it more sounds like thinning out the competition - after all, if everyone thinks you're gay, no girl is going to think they can get with you, which makes you a non-option. Given the nature of high school relationships and the people in them, though, it may - or may not - be as simple as a shitty practical joke, but still, those relationships don't last long, and they may have just been making sure you couldn't get with anyone they wanted to be with, though AFAIK most high schoolers don't have the foresight or self-awareness to come to that conclusion. It's also possible that they honestly thought that you were - as sure as you are that they knew you weren't, are you really that willing to assume malice over stupidity, especially considering how dumb high schoolers are in general - and how dumb jock douchebags tend to be specifically - over something you can't really change except to inform past crushes that no, you're not gay, and yes, maybe a date wouldn't be so bad, if they're not romantically preoccupied?
On another note, though - yeah, we've all felt like Quantum Leaping to fix one big HS-era problem that would've saved us from our current, miserable existences (in case anyone forgot my preternatural talent for shameless oneupsmanship: one GF in my entire life, lasted six years, I have a kid with her, haven't recovered mentally, emotionally, and certainly not romantically or sexually - it's been five years since then), but the problem with dwelling in the past is that the science isn't there just yet - we just kind of have to bury it, dig it up every so often for reference, than re-bury it after we've learned some sort of meaningful insight or advice to help win today's battles. It can only hurt you more to obsess over chances you might have had, because they're gone...but in time, the future will avail you opportunities, if you give it the chance ("give it the chance" meaning more than just sitting around and waiting, of course - you've gotta do you in the meantime, but also put yourself in positions where others can see you for who you are and hopefully appreciate you, advice I'd do very well to actually follow).
There's also the fact that life is long, people are incredibly numerous, and the worst that can happen if you ask out a girl you like is that she'll say "no" (or "I have a boyfriend", or become violent towards you). There's also something to be said about becoming just friends with girls as well - if you're actually friends, she/they will probably help you sort through whatever issues bar you from dating. The main thing is, you probably aren't going to die from talking to a bunch of women, and so long as you're not overly pervy or bordering on criminal, you're probably not going to get hurt or hated, either, so there's no real reason not to just throw yourself out there apart from fear of rejection (pretty strong demotivator, if I'm being honest here), and eventually, like all shitty, painful things, that'll get easier to deal with.
It also helps to know what you're really looking for - I mean, sure, you want a girlfriend because you've never been with a girl, but what does that mean? Do you seek the emotional comforts and nuances of a committed, long-term relationship, the fun and the euphoric rush of a casual relationship (which can segway into the former), near-perpetual companionship and friendship with someone who understands you more than you understand yourself, or still-enjoyable physical gratification with another person? It's not an easy question to answer, at least at first - there's so much overlap and it feels offensive to one who longs for love if he or she comes across the possibility that sex is their strongest motivator, but eventually, there are factors to consider. Do you have female friends? Are you particularly close to any of them? How many of the needs of your perceived relationship are fulfilled by "mere" friendship? Is it that you simply want to experience something you never have before because it seems to be part of what we've come to know as part of our progression through life towards becoming fully actualized people? Do you feel that everyone else around you is closer to this self-actualization than you are because of this?
You don't have to actually answer any of these questions, obviously (at least not to me or anyone here - probably wouldn't hurt to ask yourself, though), and there's a lot of me in that last paragraph (I'm dealing with a lot of this stuff - it occupies my mind more than I'm comfortable with) - I'm not going to sit here and say that this stuff will get your desired results and such, because I haven't followed much of this advice, and I'm very much in the same boat because of my non-adherence to it, but I would probably be in a better position to give that advice if I had followed it.
Also, you're 25 - you have literally decades to potentially date girls. The main thing is, the past is only good for learning from your mistakes and occasionally getting lost in some good memories - it's the future you've gotta invest yourself in, because it's the only thing you can change.