The game has a damsel in distress plot and they spend half their time together hugging. If they'd been opposite genders no one would think they were just friends.
(Edit) Though I do remember finding a journal entry about Sam dragging Lara out to meet boys at nightclubs. There could be any number of explanations for why that was there.
She didn't drag her out to meet boys at nighclubs, that's the entry:
I suppose on some level I have Lara to thank for this job. She's always believed in me. The academics at college dismissed filmmaking as nonsense. They just saw me as this ditsy American troublemaker with a camera.
But you know what? Most people can use a little trouble in their lives. And deep down, I know Lara wanted just to cut loose sometimes. She just needed a little help unlocking her inner party girl.
We had so many awesome adventures together. That insane backpacking trip through Bulgaria, I was always dragging her out to clubs. And the hiking trip on the south face of Kilimanjaro; all Lara wanted to do was explore ruins, but who knew we'd run into so many cute guys? Certainly not Lara, haha!
I have a feeling this expedition is going to be one for the books. Lara with her notebook, me with my camera... another crazy adventure!
It didn't even say that they met those guys in clubs. Now, especially that line: "Certainly not Lara" could suggest many different things, it's probably intentionally ambiguous. Some people read it as a 'proof' of Lara being into men, but it could also mean that it's just Sam sharing her thoughts about men they encountered during their journeys, and Lara was just not interested in "cute guys" (for example because she's gay), that's why she certainly didn't know they would run into them. Because it wasn't something she had on her mind, unlike Sam apparently.
Though I wonder then, if the comics openly imply they have relationship as I've read on this thread, then it would mean that Sam (who's apparently bisexual then) was interested in other people, men particularly. Or did it happen much later?