Leviticus 18:22 - Thou shalt not lie with mankind, as withwomankind: it is abomination.
For Christians this is simple logic. He accesses you by proxy. Many people do not know this.
St. Paul says "Bad company corrupts good morals" (1 Corinthians 15:30 and 1 Corinthians 5 9-11 as well as Ephesians 5:6-14) This corruption first becomes a spiritual reality even before it is visible, and it allows satan to set up shop in our hearts. Christians may not always know what ungodly things their friends think about or do- that is now always visible. If it is visible then they must have nothing to do with it-- that must be clear. And when it is not visible, at least Christians must be sure on their part that they are not in agreement with those who participate in anything that is not of God. Because if Christians are, and even if they are not doing it themselves, they get the effects as if they were doing it, because their hearts are close to those by association . And with this access, satan can block our graces still.
If Christians discover these associations they are to repent of them immediately before the Lord and cut themselves free of them with prayer.
Christians are aware of Joshua 24:15 --If it is displeasing to you to serve the LORD, choose today whom you will serve, the gods your ancestors served beyond the River or the gods of the Amorites in whose country you are dwelling. As for me and my household, we will serve the LORD.”
Read more: Why marriage can only be between a man and a woman