I tried using a catheter which is a tool made for sounding so logically it should work well.
Nope, hurts like hell. Can not recommend.
Perhaps you used a diameter that's too thick to be comfortable? Honestly one of my biggest turn-ons is to draw a guy's fresh cumload up into a catheter and then (after lubing it up thoroughly, of course) slide it down my dick until it's near my prostate and slowly force his cum back out inside me while withdrawing it, leaving me with that truly exquisite feeling of all your "man plumbing" being full of cum that happens naturally, but all too briefly, before an orgasm. I've always used a pediatric catheter, though, usually a 10 French, which I don't think is even as big around as a piece of cooked spaghetti. That makes them super flexible and the shorter length also help me from getting overzealous and accidentally reaching my bladder.
Usually I only feel a tiny bit of discomfort as the tip passes beneath my pubic bone, but the entire rest of the experience is breathtakingly pleasant and sets my mind ablaze with the intimacy of knowing I'm about to feel another guy's cum in places where typically only my own could be. Once it's all up there and the catheter is back out, it becomes a contest of wills between my desire to keep enjoying that sensation of fullness in my dick and deeper plumbing and the desperate urge to orgasm and watch it shooting out of me with my own cum right behind it. I got introduced to this by the first guy I ever had sex with and once with him I managed to hold off just under three hours with his cum in there, which was such a wild experience as I believe it liquified inside me and began to trickle back down into my balls, as they started to feel extremely warm and swollen, as well as a bit achy, but in a good way. In the few times I've done it with others as I've gotten older, I find it a nearly Herculean feat to even make it to the one hour mark.
Cumming a double load like that is its own special kind of pleasure too, as your body has to spasm twice as long to empty itself out, not only prolonging the pleasure of your orgasm but also leaving those muscles inside a bit sore afterwards, which feels so hot. It's also hot seeing the consistency change of your cum shots between his load and your own, since the extra time generally means his will be thinner and shoot farther. Overall I just think it's an amazing way for two guys to be intimate in an unconventional way, and the fact that the sensations involved are so pleasant is just icing on the cake.
Sadly, I have yet to have a real canine experience of my own despite fantasizing about it for what feels like forever. I hope that when it finally does come to pass that I'm able to get to experience this special bond with him by filling myself up with his cum through my dick, too.