There are many reasons why bribery is bad, and those apply whether the bribe is in cash in your pocket or sexual favours. I doubt you would be overjoyed to find your manager had purchased a system other than the one you recommended because the sales rep slept with them. That's just one example. Or are you in favour of corrupt business practices.

On the other hand, there are many things that it is unethical to want of another as a factor of doing business with them. These can range from turning a blind eye to illegal practices, passing along insider information and yes, sexual favours. If you were a sales rep in Singapore, trying to do your job and the client asked you to go down on him before he agreed to the service contract, would you consider it ethical of them to ask you that. And yes, I'm well aware that I am (from your post) almost certainly addressing a heterosexual male. That's the point - you're being asked to provide sexual favours to a man in exchange for certain decisions with your company. Would you be happy? Or if that's too outlandish a scenario for you (though it's a valid comparison), how about if your female partner were the rep? Or how about if you were the rep and you learned that a rival had got the deal because of sexual favours?

Basically, you have immediately put yourself in the favoured position in this scenario (and declined to imagine any sordid negative things that could well be the case here), and ignored all the reasons why this is bad - from the basic caveats against bribery to the problems of sexual favours being a commodity.

I have a hunch that if someone got ahead of you for sleeping with someone, you'd be one of the first to complain. And I doubt you would be very happy in a world where business partners and colleagues considered it acceptable to trade sex for advantages. Not if you've ever dated a girl with a job, at least. Things like this make work environments a very hostile place for women.

Is it your money?

If it is your money, then essentially what you're talking about is simply paying for sex (or rather, indulging your fantasy that being jacked off by a pair of hired labia is somehow better than just doing it yourself).

If it's not your money, but you have a responsibilty and a duty to spend it in the most effective manner for the benefit of the company/group/whatever, and you then choose to spend it less effectively in order to buy yourself sex, it's a roundabaout way of skimming from the till to pay for hookers.