Kyria, from a man's perspective, who owns his own business, this is my take.
Being involved in your company (it belongs to both of you) is very important, my wife wants nothing to do with mine and this is difficult because we cannot relate to each other regarding issues of stress and problems that occur. Anything that you do is better than doing nothing at all.
I'm sorry to hear that he does nothing around the house, this is unforgivable as in the company it's his house too isn't it ?
Does he have any health issues ?, do you see him undress ? if there isn't another person involved then there has to be something else.
People do not just fall out of love for no reason, if this is what it is.
I don't recommend this but if you are relatively computer literate, there is a program called sspro you can google it. It logs key presses, screen shots and logs websites.
As for seeing anyone else, forget it, this will be the road to hell, if it gets to this point then you should leave.
Remember if your married you are entitled to 50% of the family wealth.