Threesomes are more common with men and usually involve fantasizing only. Women tend to not want to participate in such an activity. I fear that you will participate either to please him or fearing that he will leave if you don't. Either reason will lead to multiple issues in the relationship including you resenting him, you being sorry that you did this and your feeling guilty. A person that loves you would never allow you to participate in activities that you find so distasteful. This proves that sexually he is totally self centered and think of himself only. He should not want you to do something so against what you believe him. This is not common with men beyond just macho talk. This is not normal and if it was it wouldn't matter. It should not be part of a loving monogamous relationship. You have to stay with your own convictions because once you do it you will be sorry. If he is faithful he should accept this relationship as enough.
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