A man is getting fucked in the ass. ... it's that lowkey gay?

If it's from a man and a man or a woman and a woman yes, but a woman and a man having sex, who ever is doing it, its still straight.

That's from a stigma where if a guy somehow 'discovers the pleasure of the backhole' they might start craving the real thing and not just fingers or a peg.
(๑•ㅂ•)و✧ what matters is what gender you ultimately prefer to hold/hold you at a certain time period. And if you find out you actually deviate from your original perception, you just discovered another fact about yourself and now you know yourself more a little better.