Originally Posted by bandit.45
If you take that approach you're going to be a "guerilla" for the rest of your marriage, which will probably not be long.
You need to bring it out in the open. Express the concern, and use it as a springboard to have a healthy conversation about boundaries, respect, and affair-proofing marriage. If two married people can't talk about that out in the open, there's little hope for long-term success.
Just going undercover isn't going to solve or fix anything, it will only allow him to pinpoint the exact time in which his wife wants to insert someone else's penis into her.
Guerilla tactics are necessary when you have the conversation, your wife isn't responsive, and she insists on violating boundaries. At that point, you are just waiting to collect evidence. Though at that point, the marriage is already in the death spiral...