I didn't offer any advice, but here's my take.
Seamus seems to be okay with your poly when it's theoretical, but when real life possibilities come up, he panics, immediately tells you how awful he feels instead of trying to work through anything on his own, lays a bit of a guilt trip on you, and then you have issues and feel guilty in return. Then, he calms down, things are okay, but you ultimately decide not to persue whatever option you had.
Granted, this is based on this example and your previous potential, the ex who you had the contact info for, so take it or leave it.
My personal opinion is that Seamus needs to spend a bit more time reflecting on his feelings before hitting the panic button with you.