It May be a Stretch . . .
I don't know if anyone has ever really talked about it before, (I'm sure they have) but in 8x15 "Man's Best Friend with Benefits", we see Dean's curiosity over "unique" sexual choices come out for the first time. He asked Porsche, the familiar who takes the form of a dog, about her relationship with her Witch master. She indulged him slightly by saying "You want to know which came first, the dog or the girl?" to which Dean did his adorable "No, but yes" response. We never really get an answer to that question, but Dean seemed truly interested. He wasn't grossed out or freaked out-- he was just truly interested-- almost like he was trying to learn about something for his own sake. A lot of focus is placed on the familiar's and the witch's relationship. Much of the conflict in the episode comes from it not being an accepted lifestyle among the magical community. Obviously, a theme playing onto a much larger, real-world social issue (less about animals and more about accepting people's choices.)
Which makes me wonder though, why so much of this theme of lifestyle acceptance was linked to Dean? His awkwardness is of course, always a great form of comedic relief; but so is Sam's when you think about it. The writer's had Dean be the interested one though; why make him so truly involved in how these two people/beasts carry on? I feel like in some way, Dean was relating to them. He wanted to know how they fell for each other in spite of the social pressure against it. He was asking for advice from this familiar. He wanted to know who she identified herself as more than anything-- even asking "I just wonder which you consider yourself as, mostly?" He wasn't asking about how they do "the dirty" as a dog and man, he wanted to know about her identity . . . why?
As an aside: the familiar could have just as easily been written as a man and the witch be a female. We have mostly seen female witches on the show anyway. The fact that this familiar was both a dog (something that Dean does not really like) and a female (something that Dean says he likes to the world) makes me wonder if she is a metaphor for his character in some way. He has something inside him that the world might not accept, like her dog side; and he has his outspoken-exterior, which is socially accepted, like her human side. Sam's interaction is very limited. He likes dogs (or metaphorically girls, no question, we all know that) but Dean, well, it's a grey area. In later episodes, dogs are brought up again by him becoming one, identifying with them directly--not loving them as a separate thing. Carrying on the metaphor, is he truly drawn to girls (like Sam is drawn to dogs), or does Dean just identify with them?
Well, I think that in some subtle way (especially since two episodes prior, Dean had a 'gay thing') the familiar is a shadow of Dean's own inner feelings. He tries (kinda) to not mention their unique relationship, he tries (again, kinda) to be quiet about it, for Sam's sake; but he can't, he has to ask Porsche about it as soon as they're alone. At the end of the episode, there is an understanding between the two of them. Dean even says "I like dogs" and the familiar responds with "No, you really don't."
In my Destiel riddled mind, he might as well said "I like girls." and she responded with "No, you really don't."