Let me start my saying that I am an amateur here. However, I know a bit more about computers than most folks, so I will attempt to explain what I think about this.
The behavior sounds supicious to me. Invites can be sent as spam to any email account. It has happened before and will happen again. However, once you visit certain sites and display an email address, the flood gates will open. Once a porn spammer gets confirmation that an account is active, he starts using it for each of his projects.
One thing that you can do is to examine the Internet History files if they have not been erased. This assumes that you have access to the machine. There is also an issue (from the software side) if his login does not have admin rights. Normally, there are 30 days of history kept. This can be changed via the Options command in IE.
You are now suspicious. Examine your relationship. Do you think that you are meeting his EN? Do you honestly know what his ENs are? My suggestion is to spend most of you efforts here. Will he talk to you? IMHO this is deeper than "I am curious about internet sex".
If your spouse will not talk with you and you cannot make progress, then you will have to make choices. I used a keylogger to expose my WW's EA. This is a major trust violation to her. She cannot even compare this to her violation of our trust in this A.
I will tell you that if my W saw this post, I think that she would be extremely upset about what follows. However, I feel that only the M is sacred. "Let no man put aside" is the phrase that comes to mind. I will say so long from here. Please, only read the below after a deep contemplative review of you M. Be honest with youself that you have tried EVERYTHING else.
C-
ONLY if you choose to proceed in this direction:
1. He has porn on his locked up computer. Why would installing company banned software be a problem. He should have already been fired. If this is required to save your M, then think about it. Companies have policies but VERY FEW have the resources to search and prosecute. It would tie up all the network time. Even companies that log internet use cannot have someone review all the logs. It is just too much data.
2. There are programs that can be purchased that will install keyloggers and video capture remotely. You use another computer to send a literal virus to your husband's computers. These programs are not cheap and will be several hundered dollars.
3. Programs are one way. The other is miniture video cameras. Remember, all you need are account logins and passwords and a small camera can do it. Once again, they are not free. So you will have to think about how much is your marriage worth.
4. There is a hardware keylogging device. All you need to do is to install it on the back of the computer where the keyboard is plugged in (It looks like a small gender changer adapter). This will avoid virus scanning issues that others have.