I did pick up on it but like a lot of others, I never really paid much heed to R+L
The bolded was my primary inkling. I assumed it was so and it explained some actions but, I never really considered it a big deal in the books. I actually didn't connect the Rainbow Guard bit until...just nowI'm the first to admit that I'm not the most attentive reader, but I did pick up on this! It was Loras saying to Sansa that he'd lost someone he loved that was the first clue. The Rainbow Guard and Brienne was the second. And finally, isn't there a line (I think in Feast for Crows) where Jaime and Loras are having words and Jaime promises to "stick his sword in places even Renly never found". (I can't remember the exact words, perhaps Needle or The Imp might recall?).
I gave Game of Thrones to a friend for Xmas and she totally loved it and bought all the rest of the books herself. She figured out Loras & Renly herself, but R+L=J went completely over her head (as it did mine too).
Renly did marry Margery Tyrell and never bedded her. BIG HINT!!!!!!!!!!
I remember very little of the books, but I always thought that Renly went with lots of women. Did I just make that up in my head? (Likely!) Even when I found out from this site that Renly was with Loras, I still thought he was bisexual for some reason.
My only real issue is he seems dour and pouty instead of jovial, light hearted and humourous.