@OP You'll find that many lesbians have had sex with men, and many straight people have had homosexual experiences.
This is ok, and pretty normal.
Also many people view their sexuality one way, and then are sideswiped and fall for someone who they wouldn't have considered as part of their dating pool (probably more common that previously heterosexual people fall for someone of the same sex, but it happens the other way too).
There's a bit of bi (or pan if you like) in a lot more people than they may think. Sexually can be fluid. Again, that's fine. We have too many labels IMO and I think people should be less attached to them.
But as PP have said, words do have meanings, and a lesbian is a woman who is same sec attracted; who wants sexual and romantic relationship only with other women.
What happens when we get too attached to labels is that we can over identify with them. This links to the current use of 'identity'.
A woman who sees herself as a lesbian, and hits it off with a man in a bar, or with a trans woman, and has sex because both parties want to, then good for her (remember that casual sex is basically hedonism anyway, and fun is fun, these things happen).
Whether she is 'still a lesbian' I suppose depends what happens next.
If a straight woman gets off with a female friend and then goes back to being het, she's still straight.
Consider the 'girl crush'; the straight girl who has special feelings for another women (usually a celebrity). A couple of years ago there was a wave of straight girls on social media saying they'd 'go gay' for Ruby Rose. And who could blame them?
In short, don't be over concerned with labelling yourself. Love, fancy or shag as you will.
But the problem with a couple as you describe calling themselves a lesbian couple ripples out, and blurs boundaries for other people.
Lesbians have the right to be exclusively same sex attracted. And it's ok to not be a lesbian even if you've previously considered yourself to be so.