They could have been bi or they could have been 'in the closet,' or they could have identified as something else. For example, Dax possibly was an omnisexual, or a pansexual, or 'sexually fluid' - which is sort of a buzzword, I think.
So, no, I don't think all the 'supposed' heterosexual relationships shown in the franchise told the audience the orientation of the characters. For all we know, Picard could have been bisexual. Just because we didn't see it doesn't mean it wasn't true...or couldn't come up in a future reboot...
Lt. Yar could have been a transgendered individual. Again, just because it wasn't explicit in the series doesn't mean it couldn't be true...or come up in a future reboot.
I should also add: For all the 'heterosexual' relationships that Kirk had onscreen, it didn't stop fanfic with him also having a 'gay' relationship with (or having feelings for) Spock, particularly one that was published by Pocket Books - "Killing Time," I believe was the title.