Is sex without love or romance irrational?

This is an interesting question.
While not exactly the same issue, I think it's very related to arranged marriages.
There are a lot of people from India where I work and many still practice arranged marriages. One guy I spoke with who's been married many years was pretty cynical about the whole idea of romantic love, claiming that there is only physical attraction and it always fades, but that love can grow from it.
Another who is about to commit to an arranged marriage, mentioned that what he likes about arranged marriage is that he doesn't have to "go through the trouble" of finding a romantic partner; his parents have done it for him.

Aren't you asking two different questions?

The first is "Is sex without love or romance irrational?" Obviously no, or we wouldn't be motivated to do it ;). Sex is a natural part of being human.

The second is "Is it wrong to have sex with someone that you don't love?" That is a moral question. Since I personally believe that morality is relative, not absolute, you are the only one that can answer that. In the context of Christian belief, it can be wrong, yes. I am personally not religious and, being an adult, see no reason why sex should ever be wrong. Provided it's consensual =)