aryn007 answered your question: Ok, this is bothering me too much now, and I need...
Well, Arthur’s sister is older than him. I imagine that while Gawain is younger, there’s not a staggeringly gigantic age difference.
See, that's what I thought too! I usually work out logistics issues by writing them down and talking myself through them, so bear with me here.
If Arthur is in his 20s (probably the oldest he could still be and be thought of as 'pretty' rather than 'manly'/'handsome'), let's say Gawain is in his mid-teens. Let's go with 16 or a bit older? Just like the Song of Ice and Fire series, due to low life expectancy, everyone has a ton of violent adventures before they're 18, so 16 sounds like a good age to send him off to meet his death, I guess.
Let's also say Morgause was 16 when she started having kids (and she had 6 of them?? Holy smokes!)...that would still have her as at least 32 years old (or probably well over, depending on the source and how much time passes between her marrying Lot + having his babies and her joining Arthur's court)? And Arthur is into that?
I am grateful for your reply (since it confirms half my idea), and yet this continues to baffle me, because I tend to fixate on all sorts of details until everything is clear, haha.