If you're significant other is posting pictures of themselves and flirting actively with other people on a dating site, then chances are you're not in the relationship you think you are or they're not getting what they need out of the relationship.
I'd start asking questions about the latter, and depending on the answers come to terms with the former.
There are trust and communication issues that need sorting out. You don't trust him at this point and he apparently doesn't trust you well enough to communicate his needs with you.







