Dean is either Bi or Pan.

One hint is the siren.
A siren looks like whoever you think the most desirable person is or looks like.

Dean's siren was male.

Now he also likes women.
We know this from Lisa and pretty much the whole show.

Now a little further into him being pan or bi.

He could be bi because bi means either gender.

Dean obviously likes girls and it's hinted at and joked about how he might like males as well.

Pansexual means not limited in sexual choice with regard to biological sex, gender, or gender identity.

And Supernatural never gives us a reason to really think that he would choose preference by these regards, so why not?

And now imma dive into my wide variety of ships for this show.

Have a good day folks!