Can you be bi/gay when you have dated men in the past?

Yes.

If you are physically attracted to those of the same gender then you would be gay, regardless of whether you have dated the opposite gender, if you had no physical desire towards them. If you have interest in both genders, then you are bisexual, even if you have stronger feelings towards one gender over the other.

For the record I'm a gay man, but, dated a few women when I was younger. Back then, I had not completely realized/figured out that I was gay and dated females because it's what guys did. Fortunately, I figured things out fairly soon after that, at almost 19.

Well, I know for a fact you can be bi after having dated men.