Anyone can say anything they want, but what I'm saying is informed by scientific consensus:
Although scientists favor biological models for the cause of sexual orientation, they do not believe that it is the result of any one factor. They generally believe that it is determined by biological and environmental factors; they state that most people's sexual orientation is determined at an early age, and sexual orientation development involves a complex interplay between nature and nurture. The biological factors are genetic and hormonal, both of affect the fetal development of the brain, while environmental factors may be sociological, psychological, or involve the early uterine environment. Scientists generally do not believe that sexual orientation is a matter of choice.
Link
There are plenty of sources linked there, if you want to read more in depth. There may be some environmental factors involved, but one doesn't choose one's environment anymore than one chooses one's genetics.
And no, scientific consensus doesn't mean something is 100% fact, but it means a lot more than "I think it's a cultural decision." (But still not a fact)
Not to mention, homosexuality has been observed in other animals, as well--animals that presumably don't have the chance to "experiment in college." That also strongly implies that it's not a choice based on culture.
You're misstating that. Reproduction is achieved through eggs and sperm (penises and vaginas aren't even necessarily required anymore).
Penises and vaginas are "meant for" a lot more than reproduction (Where did I say ONLY used for?). I mean, where do you currently urinate from? Not to mention sex is no longer (and probably never was) just for reproduction.
Saying that sex is only for reproduction is (your putting words in my mouth dude. I never said ONLY)A. probably not something you practice yourself, and
B. also rules out usage of human faculties for other things that aren't necessary for survival, only pleasure, like art. So yes, penises and vaginas are meant for more than reproduction.
You're free to believe what you like, of course, but the scientific evidence, the knowledge of humans actually work (not as pure survival/procreation robots) and common sense don't really support your belief, in my own opinion.