I would definitely seek some counseling for this issue. I had had many,many thoughts like this for a long time and thought I would never act on them....but when things finally got unbearable....I was amazed at how easy it was to act out these thoughts....please don't wait until it is too late.
Plus, if you get help now, you WILL NOT be put in a "nut ward" (don't like that name as I have been there, and don't like being thought of as a nut, but anyway). No one will punish you for getting help, they will just want to help you.
You sound like a very curious person and question relationships and life in general, which may be completely harmless, but don't wait until you are at wits end to learn whether or not it is serious. Please seek some sort of counseling.
LOTS AND LOTS OF HUGS!
-Angel