Would like us to be friends again

I made the mistake of getting physically intimate with a business partner, and things have been very awkward between us since then. He used to be a close friend, but he doesn’t have too many conversations with me since that incident. I don’t know if I did something horrible, but it was mutual, and I don’t understand why he is behaving the way he is. I don’t want to lose him as a friend and partner because we work very well together and have managed for many years. I know I can’t go back to the way things were, but I would like to make some effort towards normalcy and don’t know how. What should I do if he doesn’t even respond to my texts unless I keep calling or messaging? 
It’s obvious that there is an underlying issue that isn’t being addressed here. The onus of fixing this isn’t on you alone because, as you rightly point out, whatever happened between you two was based on mutual consent. I suggest you confront him with this and speak openly, because he isn’t willing to engage, and things cannot return to normal without that airing of views. Explain why this is affecting your personal and professional relationships and point out why his inability or refusal to resolve this can only harm you both. If he needs time, it’s okay for you both to distance yourselves until you are ready to have this conversation, but it cannot be avoided forever.

Is it okay to be in a relationship without having sex? My girlfriend and I just like being with each other and haven’t felt the need to have sex more than a few times because she doesn’t enjoy it. I don’t mind not doing it because I love her a lot, but my friends think it’s a red flag and tell me it’s a sign that there is something wrong.
This is between you and your girlfriend alone. Your friends are entitled to their opinion, but this doesn’t involve them.