If you want to keep them as friends I think really the best you can do for them is pray. Almost any other angle you take with them is going to likely end up weakening your friendship. What would happen in my case is I wouldn't see that as really a bad thing as long as what was said was true ( they chose the opposing lifestyle not you). If you do speak with them you need to stress that you love the person but hate the sin. This is a matter of tough love though, and they may not see it that way at all. They are living a deceived lifestyle at best and against the will of God.
It seems that accepting them requires you to accept everything they do. But that's not how it works generally. You accept relatives even if you do not accept all that they do. You accept neighbors even if you do not accept all that they do. The homosexual agenda is that we must accept and affirm the goodness of homosexual acts and anything else means not accepting them as human beings, being ourselves haters worthy of death. If that is what these people believe they may be permanently closed to hearing anything about Jesus. But if, as you say, they are friends then maybe that has value and they are not pushing an agenda. Not all homosexuals actually are pushers of that agenda. So explain why you value them, as people made for the express purpose of loving God, knowing God, and serving God in this world and the next. People who your savior died for, out of love, just as much as for you. It may be very uphill, but if they see that the first idea out of your head is not to hate them, which it obviously isn't, maybe you can get somewhere. You can stand firm and be a friend. There is a slogan 'hate the sin and love the sinner'. There is a flip side to that, unknowingly practiced by many, which is 'love the sin, hate the sinner'. That's what people do when they accept behavior that is truly unacceptable. That's not really love, even if it is tolerance. We should be intolerant lovers.
If they "ask"
There's probably a good chance that your friends already know that many Christians are anti-gay. I'd say that this is an opportunity to show your friends the other side of Christianity. Show them that Christians can be kind and generous. If you talk about your faith, tell them what your experience of God is like, how Jesus has forgiven you, how the teachings of Jesus inspire you to live a better life.
You defense then against forbidden is to elevate the whole church to the same level as forbidden sexual immorality? You declare the whole church as unrighteous to justify unrighteousness.
