Well uh…
1.) one or both of people you see as a “straight couple” could be pan/bi/poly/ace
2.) one or both of them could be trans or non binary
3.) you could be misgendering someone
4.) They could be there to give moral support to a queer friend or family member who didn’t want to go alone.
Number four is important
5. They could be there because they support the cause stop fucking gatekeeping
6. They could be there in memory of a loved one, don’t forget Pride used to be a memorial as well as a celebration. I know a good number of straight people who go to Pride to celebrate the lives of friends and family who have died because they want to remember them as they lived, happy and joyful and surrounded by a community that loved them.
ALL OF THE FUCKING ABOVE.