I don't agree that Sean's feelings for Des should overcome any awkwardness on a hometown date. That's the whole reason for hometown dates. You want to see where the person came from and if you could see your families melding. As one of my sons-in-law once told my youngest daughter, "You really don't just marry the person. You marry the family." I know that relationships have been and can be successfull when inlaw relationships don't work, but if you are looking for defining qualities to know who among a group is right for you and one family member made you very uncomfortable...I can see that imagining Thanksgivings together would give you pause, even if you really cared for that woman very much. And remember, Sean has said how important family is to him. He is very close to his own family and would have the expectation of being close to the family of the woman he chooses. The whole Romeo and Juliet scenario sounds romantic, but in practice, family harmony is important as it has been cited as a major cause for marital strife.
If Sean didn't really care for Des, the decision would've been much easier. Since he needed a reason to send someone home and he had such an uncomfortable evening with Des's family, it prompted him to make the choice he made.