Initially, I thought it was bad. I had a blanket rejection of the whole practice. But now I don't. If it's done in the right spirit and with respect.
There is the odd guy who will just effectively wham it in your face whether you want it or not. Of course that is not good.
But there are many guys who, after a connection and proper bit of conversation, ask if they can because they just want to share it with you. And then suddenly it starts feeling like a privilege of some sort, I respect it, and I know I have a choice.
When you think about it, if you are having an openly sexual conversation with a guy, and it is having a noticeable effect on him (...), then it kind of makes sense that it is a very obvious and tangible part of his arousal, and if he feels close to you then he's going to want to share it probably.
So my opinion is:
Unsolicited - no.
Offered after a connection - acceptable.
The inexperienced perspective is that this is some kind of flashing. But once you experience the depth of a mutually sexually arousing/comforting virtual relationship, when you are in that zone then it isn't flashing at all. I mean, if you're having sex with a guy, you would expect to see his dick, wouldn't you?!