I'm so sorry @beckstar2021 - I really can see how upsetting (and grim) this is for you. I don't think you are being unreasonable.
My experience is based on witnessing a little of this but from a somewhat different perspective. My lovely dad died very suddenly and tragically, no warning whatsoever, when he was just 70. What struck me was how quickly a number of local widowers, who had known my parents socially, started 'honing in' on my now-widowed mum - one within a matter of days. M is quite naive in these matters, and chose to see it as nothing more than local friends being extra-friendly and supportive. But it was very clear to me and my DB and DS what the intent was; to the point where we discussed potentially having a word with the most persistent caller, before he seemed to take the hint and back off. One of them even mentioned "having someone in my life again now, to do the cooking and cleaning". I get the fact that time is increasingly of the essence when one is older, but really...
I don't think you will be able to say anything to make him stop, but you can do your best to opt out of hearing the salacious details. As soon as it starts up, you can exit the conversation "sorry dad, someone's at the door." Or just "have to go now dad, bye!".
One thing I would caution you to be mindful of, however, (if your dad will listen; possibly not, but you can warn him and be watchful yourself) - if he is putting himself, his picture and his details on socials and dating sites, then he is opening himself up VERY widely to scammers. Scammers, particularly romance scammers, prey on the vulnerable, lonely and desperate and your father's actions are the very pointers they seek out. Be on the alert for your dad potentially sending money or Amazon/Apple gift cards to "girlfriends". Hopefully this will not happen, but it can and does, even to intelligent and tech-savvy people, so if you think it may be starting up with some of your dad's correspondents, it might be worth contacting his bank to report any concerns - Romance fraud - Report Fraud. I really hope that this advice is not needed.
I wish you better times ahead, and I hope your dad can find peace and happiness in his own way. I'm really sorry you are navigating this as well as the loss of your mum. x