Friends and partners

Hello!
I would like to ask if it is necessary for someone to meet their bestie's partner?
I have had a friend, we used to spend a lot of time together, we talked about our secrets, we had lots in common. About 6 years ago he had a boyfriend. I supported him, I made friends with his partner, we spent a lot of time together. However, I felt very pressed because I did not have a lo in common with his boyfriend, I did not enjoy his company ...but I couldn't say anything to my friend because he was in love and I was afraid that he would be mad and get away from me. As the time passed by, things got worse, I started to have stomach aches and panic attacks which led me to a psychotherapist who advised me to talk to my friend and tell him that I do not want to spend time with them. I could not do that to hurt my friend so I preferred a middle solution.. I told him that his boyfriend is very nice but I do not have lots in common so I do not enjoy all the times with them, so I suggested that we should soend time together, go on trips etc but not all the time, because we did everything the three of us. My friend got really mad and disappeared for almost three years. He did not approach me even when I got seriously ill but he returned when he broke up. We started our friendship again but I promised to myself that if he has a new relationship I will not get to know him because I do not want to pass the same thing.
We spent about 3 years as good friends and six months ago he met a guy and fell in love with him. I told him that I do not want to meet him explaining why and from that time on, he is ok with me at work, etc but he never asked me to go out with him, he started going out with other people, he stopped talking to me about serious things etc, he is distant... I tried to talk to him but in vain.. I feel sad but at the same time I feel tired to do anything... I feel that he uses me when he does not have a relationship and deep inside I do not want to be friends with him... at the same time I miss him a lot but I cannot pass through the same situation if I meet his boyfriend and I will not like him.. by some things he has said to me about him, I do not like some traits of him and I am afraid...
so, I wonder.... why isn't ok for someone to combine friends and partner successfully without being friends all together?