There seems to be a misconception that homosexual people are unaware Christians think their lifestyle is a sin. Calling out homosexuality as being "sinful" is almost universally understood by most homosexuals in the world who are familiar with Christian ideology. The real issue is that they don't perceive it to be a sin, where as most Christians do.
Reminding a sinner than their sin is sinful, seems redundant and ineffective. It's a superficial and not so efficient method of converting a homosexual lifestyle into a heterosexual lifestyle. If simply saying, "practicing homosexuality is a sin" helped people to NOT be gay, then there would virtually be a 0 population homosexual community. Obviously, "reminding" them of their sin isn't going to do much in the grand scheme of evoking change in their lifestyle.
So what is the better alternative? Show them love. Love one another. Show them how much God loves them. Remind them of Christ's sacrifice that He made on our behalf so that we could live together with our Father in Heaven. Pray and ask the Holy Spirit to speak through you to soften the hardness of their hearts, so they can be convicted by the Spirit of God and can see the error of their ways. If, by some small chance, they are unaware that their lifestyle is a sin, then of course explain why it is sinful by showing him/her the word of God. If they already know you think it's a sin, and all you do is say, "it's a sin!", you run a high chance of turning that person off and hardening their heart even more. They know this... tell them something they may NOT know... Even this isn't a guarantee you'll win them over, but you at least increase your odds significantly.