Up in the
anal sticky (what a great phrase that is!) there's a couple of discussions from guys about being soft while getting fucked.
There's two issues here:
Physiological:
The great thing about porn is that it is inspirational. The bad thing about porn is that it creates misconceptions- like, for example, that bottoms are always hard while getting fucked. They don't. The camera angles and lots of editing make it look like the bottom is hard all the time. The truth is that a lot of bottoms are bottoms in porn because they can't stay hard the whole shoot.
Some guys stay rock hard while getting plowed and some guys go soft while taking it up the ass. Both are enjoying it equally and it doesn't make a bit of difference whether your dick is hard or soft as long as you're in the moment and having a good time.
Emotional/Psychological:
The real issue is not your dick, it's your boyfriend's insecurities.
To his credit, he probably wants you to come first so that he knows he's pleasuring you or maybe he wants the two of you to cum at the same time. He wants you to enjoy what he's doing for you. Only he wants your dick to be what tells him you are having a good time, instead of just taking your word that you're enjoying it.
The problem is that when things don't go the way that he expects, he makes it all about him. The way he looks. The way he is when he gets fucked.
So, what to do... what to do...
What you can do is be more verbal. Your guy needs his ego stroked a bit. When he's giving it to you, give him lots of feedback.. the "deeper, harder, oh your cock feels so good inside me" kind of feedback. If you enjoy kissing, then tell him to kiss you while he's fucking you. And when it's all done and you're cuddling together, tell him how hot it was for you and how you really liked having him inside you.
As for the coming issue...
If Viagra didn't work for you (and I don't recommend it as a solution), then it's pretty much predestined that you're not going to stay hard or cum while getting fucked.
Instead of focusing on what you can't do, figure out what you
can do.
If kissing is what really turns you on, then maybe the solution is for you to jack yourself off while kissing him. When you're close to coming, then sit on his dick and blow your load with him inside you.
Or maybe alternate- one night let him fuck you silly in every position possible then on a different night, let him fuck you for a while then switch to what gets you off.
This way you get what you need and he gets what he needs.
I suspect you're going to find that this isn't about your dick, it's about giving him more feedback and rewards for what he's doing for you. He just needs to focus more on the expression on your face and the words coming out of your mouth instead of expecting that your dick is better at expressing what you're feeling.
maybe if he can do that, it helps 
