On the subject of "Homosexuality":

Did you see any of the heterosexuality in Gundam as pandering to horny losers?
Why does homosexuality have negative connotations to you? Maybe you have some homosexual friends who can answer questions you have about homosexuality.

On the subject of "They're like, how old?"
In my highschool most students started having sex when we were 13-16. And that's in America, where the Age of Consent is 18, but teens all over the globe have underage sex all the time without the knowledge of their parents, and also without some weird, manipulative/criminal adult intervention. Teens going through puberty experiment with their bodies and have sex. That's why Sex Ed is taught in middle school, because even before sexual education was taught, teens were having sex and having unwanted pregnancies, and other extreme consequences.

This is an uncomfortable subject to most. However, if you're open to facts, and expanding your knowledge, a quick google search would tell you that across the globe, that the age of consent ranges from 12 to 21 years old, but that in MOST countries -the average age of consent is between 14-16. These are generalizations gathered from datapoints, don't if/what/or/but them, it's a generalization. Sex in eastern media (in any context) also isn't usually stigmatized or taboo. They have a much more general understanding and acceptance of puberty and sexual curiosity, usually exploring the subject in a comedic or innocent way (Goku+Bulma in the original Dragon Ball is a great example of this).

So if you step outside of yourself, and examine these themes objectively, the real issue here isn't the content of the show, it's the context of your Western/European biases, which is totally fine and normal. I'm not judging you and no one should, you just like everyone are a product of their environment. I'm simply stating that when you consume Media from outside of your cultural bubble, you need to take into consideration that the cultural context might differ from yours, and that being curious instead of critical would better serve you.

It's okay and completely understandable that someone would pull back, and have an extreme reaction when it comes to these cultural differences. But it's also ON YOU, not other fans, not the people who created the show, and not people of that culture to educate yourself on how these topics are perceived where the media was written/conceived. And I hope you do! because most Foreign Media is more enjoyable when you do.