Thank you all for replying. I didn't even think about the practical situation, marriage, money, and so on. Luckily I can provide for myself, I have a good job. I love living on my own. Living together with my friend isn't something I am considering right now.

Jealousy is what concerns me most. Right now my friend doesn't have another partner, but it doesn't mean that will never happen. Can I handle it when he meets someone else? I think I can, but I am not sure about it.

My friend is convinced that having another relationship will only strengthen the one you already have. He says that monogamy restricts people, makes them feel guilty when they fall in love with someone else. I have to admit that during my marriage of 12 years (I divorced almost two years ago) I was sometimes attracted to other men. I always fought those feelings very hard. But still I felt guilty, I felt I betrayed my husband and our love, although I loved him as much as ever. Sometimes I wonder how it would have been if he we had had an open relationship.