A few notes: @ @Jamalisms
#1. Knight of Flowers was supposed to be the "next Jaime" in terms of skill and swoon factor. GRRM only alluded to him being gay in the 3rd book where IIRC Jaime threatens him saying something to the effect "I'll stick a sword so far up your ass Renly could never find." He also willingly takes the Kingsguard (they are supposed to be have no children, no wife, etc) at the age of 16 or so, just like Jaime. Jaime did it to be close to his lover/sister. Why would Loras?
I do wish they made it a little bit more ambiguous, but HBO is gonna HBO.
#2. Robb married Jeyne Westerling, daughter of a Lannister bannerman. This happened after he was wounded during his campaign to sack Lannisport. She treats his wounds, gives him some nookie, and he decides to do the "noble" thing. He weds her instead of leaving her like he believes his father did with Jon's mother.
HBO wanted to make the character more exotic, just like they did with Shae. In the books Jeyne Westerling is a normal, cute, shy girl. Also, If they made her a Westerling in the tv show, they'd have to introduce Raynald (the father), and a lot of the intrigue surrounding a Lannister bannerman helping a Stark for no other reason than his daughter slept with him. Without knowing what happens next, it would seem implausible to see why he would allow it. Lannisters always pay their debts, and Raynald would know that his family would end up like the Reynes of Castamere.
So to bypass all of that, instead of Jeyne Westerling, you get Lady Tylisa from Volantis. No ties to Westeros, no family, no further intrigue. It makes the story simpler, and easier to keep track of.