In his response to this issue, Dr. Salah Sultan, said the following:
“Since they are not in a Muslim country, Muslims have no other option but to advice them and invite them gently to give up these sins they are engaged in. If they responded positively then they should be told to make a contract of marriage to change this relationship into a halal one. We cannot label them as kafir because of their sinfulness; that is not the attitude a Muslim should take towards people who are not following the right path.
A Muslim is supposed to follow the example of the man mentioned in Surat Al-Kahf as having seen his friend disbelieved, he started questioning him gently:” His companion said to him, in the course of the argument with him: “And his believing companion replied while conversing with him: Do you disbelieve in the One Who created you from dust, then developed you from a sperm-drop, then formed you into a man?, But as for me: He is Allah, my Lord, and I will never associate anyone with my Lord in worship” (Surah al-Kahf Q:18 37 – 38)
So he argued with him gently and started inviting him to give up disbelieving in Allah. So we should take the approach with the people mentioned…”