It's possible that I'll win the lotto tomorrow. Obviously gay men can have children. But all things equal, when a gay man with fair features and a fair woman with a known lover have dark-featured children, all signs point towards bastards.
I'm not argueing that they aren't bastards. I'm saying that we can't be certain 100%.
I think, going by the details in TRP, it's unlikely that Laenor is the biological father, but it is speculated that Rhaenyra joined in with her husband and his lover. Quite possible to conceive then....
What I find a bit suspicious about TRP, is that whenever something bad happens, it is viewed from the point of Rhaenyras affairs. Look at Laenors death, for example. Who would have profited from his death the most? The greens, since they could now try and convince Viserd to marry Rhaenyra to Aegon (something which they tried to do). Yet the greens aren't in the list of suspects for Laenors death, nor are they on any other list. They should not be blamed for everything, but they should at least be considered.
If Gyldyan, or any if his sources, who seem to have been alive during Viserys' reign, had described Harwins appearance, and the three boys had looked just like him, than that would have been very convincing. But Harwin isn't described at all, and I wonder, is that because at he time they didn't consider him important enough? (a bit u likely, due to the rumours about beig Rhaenyra's paramour) Or is he bot described because there is something more...? Is it perhaps because he looked nothing like the three boys either?
There is no reason for Glydyan to have chosen the side of the greens (the guy lived 100 years after the Dance), but if his sources had their preferences, (and his sources were alive back then), that might explain why..