(Possibility: Randall goes out with Angela because he's in denial about his feelings for Henry.)
Dating Angela wasn't really a lie. He did like her. He'd liked her since they were kids, and they'd always played together and talked together and had even played pretend that they were married once, and his parents thought she was ok, and she acted like a real lady. She was fun to be with even if she nagged him sometimes, especially now that they were dating. And now that they were teenagers, getting to be real actual grown-ups, it made sense for him to grow up and get a girlfriend and stop spending all his time playing around with Henry. He was really starting to hate spending all his time with Henry.
It wasn't that Randall disliked Henry at all. He was starting to think he liked him too much.
Henry had this kind of girly look, with a really sweet smile and long eyelashes, and he was restrained and polite and quiet like a girl should be, and Randall thought it must be confusing his subconscious or something, because he couldn't stop thinking about him. That was what had given him the courage to ask Angela out at last: a lurking feeling of weirdness, like there was something in the back of his head about Henry and his girly looks that he didn't want to think about. Something creepy and strange.
Randall knew no one else could see those guilts and doubts that he carried around all day, but still, as he lay awake at night thinking about the soft whisper of Henry's voice in his ear, or the delicate way his eyebrows came together when he was troubled, or something else stupid and little that he shouldn't be able to remember, it was nice to have Angela to think about. He'd call up some image of her and maybe imagine her like a pin-up girl or something like that, and push Henry away. Maybe, he thought, he had been confused about who he'd really liked all along. All along, when blond hair and long eyelashes and skinny legs and gentle hands had captivated his adolescent mind, it had been Angela he was thinking of.
So he threw himself into romance with Angela. Told her his secrets, rolled with her in the grass, bought her anything she said she wanted, and even planned a grand wedding that they'd have someday and Henry could be best man and wear a really nice suit for the first time in his life and he'd look amazing in it…actually, never mind, Hershel could be best man. He'd be more reliable.
He still spent some time with Henry. He always scheduled his dates for the nights when Henry had fewer duties. That way, he and Henry could plan expeditions and talk archaeology and then he could dash off to Angela still full of the excitement and unleash it all onto her. He never kissed her harder than on those nights after Henry.
It's because of the archaeology , he told himself. He'd flush as he told her about relics and ruins and how brilliant Henry was, and she loved how much he loved archaeology.
So he kept talking about it, every time he was around her, and he tried to be the best boyfriend he possibly could, and he was busy and breathless and thoughtless around her, and he loved her, he swore he did. He did. Really.