I legitimately think the first video game to ever say the word "bisexual" out loud was Metal Gear Solid 2: Sons of Liberty. There is a nonzero chance it was actually the first video game to print "bisexual" in a text format.

It is very possible that the first video game character to ever say the word "Bisexual" out loud was Solid Snake.

Based on what I found, the first ever bisexual character in a video game was Curtis Craig from Phantasmagoria 2 (1996) but I have yet to find out if he says the word "bisexual" in the scene where this is revealed

I once again have to point out for those who've never played mgs2 he says this when asked if another character that goes by the name of vamp is a vampire

Every instance of this screenshot I have seen, I assumed was an edit as a joke.