The marriage bond and intimacy is supposed to be something special that you share only with your husband/wife. I've seen a lot of people on these threads say or indicate that sex is only to produce children. I cannot understand that point at all and have no idea where they get it from. I've never seen anything in scripture that backs that idea up, and never heard anyone in real life even suggest such a thing.
Anyways, married couples should be able to be closer to their spouses than anyone else in all ways. Physical intimacy is an expression of that and should be enjoyable for both spouses.
I'm sure that was God's intention. Yet, like everything else people have gone against God's plans and purposes and have made sex into something dirty and to be ashamed of. If it is done wrong outside of marriage or in any of the other weirdo ways going on nowadays... like gay/lesbian, stuff, etc.... then yeah it is dirty and shameful. But it should never be considered that way between a husband and wife.
However, if you are uncomfortable and feeling that way about it then you should really seek the Lord about it. I'm not knocking you or trying to be demeaning in any way, but if you are a healthy man and you feel this way then there is a problem and something is wrong. Maybe it is that you are not comfortable with your own self...IDK but like I said you should seek the Lord and really put some thought into where the problem lies.
Your statement about wishing another man could take your place sexually with your wife indicates that you do not understand the sacred bonds of marriage. Having someone cheat or have sex with another person is one of the most hurtful and devastating things they can do, and really one of the only reasons I can see that scripture even justifies a divorce. There is a reason for that.