I think it's pretty safe to say that the common understanding of the word sin is that it's something bad. Something morally reprehensible. A sin is a fault; a lack of goodness or purity.
Based on that I think it's 100% homophobic to think of homosexuality as a sin.
I agree on definition of a sin, that it's something bad/evil or morally reprehensible/lack of goodness and purity.
In a way, someone can consider it's sinful for themselves to be homosexual for their own reasons. However generally, it does sound homophobic to say homosexuality is a sin. I think sexual abuse is a sin, or violating someone's sexual boundaries or trying to control whom another person chooses to have sex with. Sexual acts themselves, straight or gay, aren't sinful. Nor is sex as an act of mutual love and desire. Cheating, having sex for money, stuff like that of course can be bad, but then circumstances...
Cis gay couples can't reproduce naturally, but do people need to reproduce to justify sex or their expressions of sexuality? We have too many people on Earth already. Imho sex is physical expression of love of one another, even while some do it with for sheer enjoyment with no strings attached. Why would homosexuality, as an act of love or expression of sexual desire, be any different than sex/same expressions between a man and woman aside from reproduction?
I think homophobia is a problem if it's used to control who other people share their beds with, or harming gays for their sexual preferences. Homosexuality in itself isn't a sin imho, but rape be it between gays or straight people would be a sin. There are some examples of adult men luring young boys into sex because their women don't satisfy them. "Boys are smoother than adult women". perhaps this has something to do with association between homosexuality and sinning, but also culture of fear. Back in the history, alpha maleness has been a huge thing, probably still is but in more controlled manner. And there is always that bunch who see that someone elses manliness/dominance is away from their alpha male status - competition in its toxic forms.
Sure, if a straight person refuses to have sex with a gay, that is totally okay as it's
maintaining personal sexual boundaries. Just as much a gay can refuse to have a person with another gay/straight person they don't like... why? Because you can be attracted to more than what a person has to offer physically, think about personality and all that, and how that affects their attractiveness physically, and whether you want long-term or short term relationship - would one have sex in short term relationship or save themselves for long-term partner(s)? Not every straight person (man or woman) wants to have sex with every other person of opposite sex they meet either.
Religious arguments generally are pretty much outdated in our modern society. I just don't think they fit as they once would have. There isn't probably a group, straight, religious, atheist or LGBT which doesn't have their healthy and abusive people within them. Can you see a person's genuineness or abusiveness from their faces or group associations though?