How would that have any effect on the person whose semen it was (and who I would assume had finished with it, given that they didn't take it with them as a few of us have seen on other posts further up)?
I think the problem lies here in that they would be making a reasonable assumption that their ejaculate is going to be disposed of, not used for someone else's enjoyment.
I personally think it's akin to someone secretly recording me and then wanking over it. It's not something I consented to. The law around recording people states that it can't be done without consent where the subject has "a reasonable expectation of privacy".
Anyone who does record secretly no doubt justifies it to themselves as "what they don't know won't hurt them" - but if I found out someone had recorded me in secret I'd be fucking fuming.
I did have a client who asked to put a discarded condom over his cock and wank into it. I allowed it once, but afterwards I felt I'd done the wrong thing and when he came back and asked for the same again I said I wasn't going to do it and explained why. I'm not sure if he took my point but he didn't argue... although I've never seen him again, so perhaps he found someone else happy to accommodate!