Different strokes for different folks and all that jazz
I just don't see how it encourages replays.
If I was deadset against playing a certain race, that race having exclusive access to an LI isn't going to be enough to change my mind.
Alternatively, if I was deadset on romancing one character and it turns out that character is only available to a race I didn't initially plan on playing as, I'd just play as that race to start with.
Now, I'm going to play the game multiple times as multiple races, but I don't see this as being the make or break(and in fact I plan on purposefully avoiding any racially exclusive romances). At best, it's one reason on a pile of them.
As for realism, give me the human/elf/dwarf companion who'll only romance qunari because they have a size/amazon fetish. Give me the companion who refuses to romance the Inquisitor at all because we're fit and they like 'em on the chunky side. Gimme the human who'll only romance dwarves because they're just the right height and the elf who won't romance humans at all because of every reason xmegaplex has. Hell, gimme the human who won't romance a human inquisitor if we select the darkest or lightest skin tone, or the one who won't romance us because we selected a mustache or hairstyle they don't like.
Put all those in there and then I'll consider it realistic.