Hi there
Really sorry this has happened, it must be have been an awful shock. He may have done nothing about it at all. It may be that he is frustrated by his erection problems and this is somehow an outlet for those frustrations. He may be fantasising only.
My dh has had erection problems from time to time. It really interferes with a man's self esteem and view of himself as a man. It's nearly always psychological in origin. I know how demoralising it can be.
It may be that he can't handle sex in a RL relationship for some reason, only impersonal encounters. It may be that presenting himself on one of these sites as a virile 'up for it' guy he's just enjoying a virtual alter ego where he doesn't have these problems.
He may be bi-curious and ashamed about it, that's why he's looking into contact with couples, IF that's what he's doing.
Whatever, you have two issues to deal with here - his sexual problems in your RL relationship and what he's doing on this site.
I know someone has recommended that you don't root around secretly, but I think I probably would be very tempted. If you respond to his message you can get a handle on what his use of the site is - would it extend to meeting up.
I you knew this, you'd feel in a stronger position if you confront him and you would know for definite if when you did this, and he denied it, he was lying. And then you can work out where you go from here.
You need to get to the bottom of why he's on this site, but can't handle his sexual relationship with you. Only other thing I have to say about this is that I urge you not to take his erection problems personally as a reflection of your own desirability. It's likely that these issues are very deep-seated, nothing to do with you.