Ah, that is much clearer. Sounds like the issue is the D gives Seamus the willies - he doesn't appear from your description to be unhappy about you pursuing men he feels treat you well and are safe. So why does D bother Seamus?
I've found that my wife has much better people radar than I do. I consider it when someone bothers her, even if I don't get the same feeling and if she can't put a finger on why someone feels sketchy to her. Often I go ahead with my interactions with that person but not always.
It's possible that Seamus is overreacting to something from the past, or he just reacts badly to D. It's also possible that he is picking up on something that you've overlooked or glossed over.
Of course, I would agree that Seamus doesn't have the right to pick your friends - and if you think about it and decide that you are comfortable, pick up the phone. I have a few people like D in my life - people who were important to me and we've lost touch - who I would like to run into again someday.