DH has a particular fantasy and is really getting me down. He fantasises about me going to bed with someone else. Every time we make love it’s all he talks about before, during and after, and now I’m beginning to dread having sex. I’ve tried talking to him about it but he says it’s only a bit of fun, but it’s really not fun for me and I actually am turned off by it all. He is adamant that I will do it one day, but I really hate the very idea. Any advice on how to deal with this?