If you are near Borough Tube / London Bridge, you'll find most (of not all?) of the pubs are pretty gay friendly. The Gladstone on Lant Street used to be gay, and you may see it listed as such, but it no longer is. I still see a few male couples in there who I would guess are gay. (For those who saw the revolting White Cube look at the GLadstone, that's gone and been replaced with a red wall, dark wood suburban home with pretensions of wealth look - not very interesting but certainly a bit more relaxing).
i guess i would say that a mixed gay/straight venue is one that lesbians, gay men etc feel safe and comfortable in and would have a significant number of lesbians and gay men using this venue as opposed to be just a couple in there. a mixed venue would be just that, mixed.
yes i would agree that it is just a pub, although feeling welcome/ unwelcome may affect if you would return.