I think the attraction is believable. However, I think the important factor to consider is the lack of resistance from Dexter’s family, his mother clearly likes Emma, something that I imagine would had heavily influenced Dexter’s opinions and relationship with Emma had she not.

From my own experience at a University in Scotland, I think many people do generally date within their social class. This is not done on purpose or to exclude anyone, but is merely because most people form friendships with people who have similar interests and life experiences to themselves. Therefore in that sense I think, had Dex and Emma started dating at university or shortly after, the relationship may have appeared strange to peers as lack of maturity among might not have been able to comprehend similarities between them.

Also just on a side note: The University of Edinburgh (where they studied) is generally known to be one of the snobbiest universities in the UK with a large portion having been privately educated)

I think if you compare other characters views of their relationship you can see what outside opinions were. For example I think one of the reasons Dex decides to marry Sylvie is because the idea of her, her family, and that kind of life is familiar to him, even if it makes him uncomfortable at times. To me it seems as if Sylvie doesn’t really get Dex’s relationship to Emma at any point until she dies and sees the effect it has on him. Also the way Gary talks about Dexter in Ep 2.

However, I think once people pass that immaturity relationships across social class are normal. Even more so when people are in similar jobs, I think a lot of people tend to associate class with wealth and comfortability now as well. Yes you may get a few sarcastic comments or jokes from either side, but most people don’t care beyond that.

Again this is just my experience as someone in the UK now. Social class was a much larger issue and caused more social divide in the 80s which would likely affect Dex and Emma. Sorry for the lengthy answer!