Perhaps they aren't as strong as you think haha. Sex can be considered quite the full body work out. Even if you consider yourself "physically fit", which most men believe is in terms of superficial outer muscles (the ones that we see) and how big or cut they are. There are many other muscles other then the visible ones that guys have a poor tendency of noticing to work out. Abdominals aren't the only muscles of your core, nor is it considered the most important.

I agree. You're probably just having really good sex lol.