Meeting someone on a dating site and worrying he might use it again

Meeting the one on a dating site and having trust issues of him using again. 
Met the one on POF, he's amazing. We're falling in love fast, both of us, but sometimes I worry that he'll be using POF one day behind my back. Why am I having trust issues if I know he might do this. I just want to know if anyone has felt the same or had experience similar. 
We're both in love right now!