I sometimes hesitate to put ideas into people’s heads that may be totally wrong, but I do sense the possibility that he is gay rather than straight, given his lack of interest in having sex with you and the fact that when he does have sex he only wants to do it doggie style. Of course there could be many other reasons why he’s acting this way, including a health problem, worries stemming from work, physical changes that have occurred as he’s getting older (though 41 is a bit early for those) or even, as you mentioned, that he’s having an affair.

I don’t think that you should ignore this situation. You are still very young. If this marriage is not going to work out, you should find out sooner rather than later, especially before you have any children. So talk about this with him, and perhaps see if he’d go with you to see either a sex therapist or a marital counselor. But do whatever it takes to get to the bottom of this.