[Quiet Man Cometh]

I'm no expert on romance or relationships, but it seems to me you're making this more complicated than it really is. Sean 1 appears to only be interesting in having an f* buddy. If you won't want that, then tell him no and break it off.

As for Sean 2, it's a risk that happens every time I suppose. Either you stay together or you don't. Relationships don't always work out. You need to decide whether you think it's worth taking the risk that it might not last.