Tell him that everyone has different ways of expressing love and intimacy. Just because it's different from what he's used to doesn't mean it's wrong. Remind him to be open - minded.
You can comfort your friend by explaining that love and sexual relationships come in many forms. The gay sex he saw is just one of them. Encourage him to learn more about the LGBTQ+ community to reduce his shock. Maybe share some positive stories or experiences about acceptance and diversity with him. Also, let him know that it's okay to feel a bit surprised at first, but he should try to move past any negative feelings and be more inclusive.
First, let him know that it's normal to be shocked when seeing something unexpected. Then, you can say that in our diverse world, gay relationships are a natural part of the spectrum of human sexuality. Share with him that the most important thing is to respect others' choices and identities. You could also suggest that he might want to talk to someone from the LGBTQ+ community or an expert in sexual diversity to gain a better understanding. This way, he can gradually overcome his shock and become more accepting.