is/was

I think you just answered your own question.

If you are describing a past situation...

Last year I had an affair with a woman I met in a bar. I didn't know she was married.

If you are describing a present situation...

I was surprised by what you told me about John. I didn't know he is gay.

Although many native speakers will use "was" in the second situation. It is not "wrong".

Re: is/was


You can use either, Tanguatlay, because, the 'was' is merely a backshifted preterite/a backshifted past tense. This means that it isn't a real past tense/preterite form, as in,

"I was in London yesterday".

It's only being used to report something previously heard or thought about. We know that it is being used ONLY to report this, because it doesn't describe him as a "former gay", someone who was gay and is no longer.

It carries the same meaning as when we use the present tensed form 'is'.

Are we talking about Slappy?

Was he in London yesterday?

Who is Slappy? Was s/he/it gay?