I would be more then happy to help you with this if you can explain some more on your relationship.
How long have you been together?
Where was he before he came home? (Work...was he late)
Has he ever cheated before?
Any other suspicious things happen as of late?
Do you want to stay with him even if you can prove he is cheating?
Well based on the situation and the condition his pants were in there is a good possibility that he has not been faithful. The fact that he is not being honest is a big issue as well. In the end he may never be honest about it. The question is can you live with him knowing he may have been unfaithful?
I can tell you all day long about statistics and how he is unlikely to change unless he wants to, but in the end what I think matters very little. It is what you think, and what you want that really matters.
I can tell you with certainly that unless he is honest and willing to make a change that he will not change. If you accept it, then he has no reason to be faithful to you. Its just like a child........if the child sees cake on the counter and he knows he is not allowed to have it but he sneaks into it anyways and mom never punishes him then he will sneak into it again. Sometimes adults can be child like in this sense..........if we can have what we want with no consequences then why give it up?
The big question is what do you want? Do you want to stay with him and ignore the behavior or do you want to leave. Would you rather try and work it out? You deserve to live your life with someone who you can trust and love.
What I would suggest is sitting down and making a list of what you want out of a spouse and partner. Then compare it with your husband. Once you have done that you should be able to decide what you really want and need.
So you have a couple of options, the first is to sit down with him and let him know that you know what he has done. Let him know that you love him but will not tolerate him being unfaithful to you. You can insist on counseling or a break for a while to see how things go. There are several different routes you can take. You can leave, or insist on counseling and work it out. Or you can try and take a break from each other to clear your head.
Your other option is to let him get away with it. I will keep this short as you really deserve better then to be in a relationship where he has so little respect for you that he would cheat on you. While this is a option, I would not suggest it. The fact is he will continue and you will come to resent and hate him over it. Eventually the relationship will come to a end.
I wish I could tell you what is the best route.......but I am not you, and as much as I see what should be done I can not tell you to do it. Only you can decide how you wish to handle it. I can tell you there are many a men out there that would be happy to have a chance to be with a women that would treat them well. Leaving yourself in a marriage that one is not a member of is not fair to you.