Yes, no problem at all.
It erases the potential of 'cheating' in many occasions and avoid arguments that way.
It makes your partner less horny in case you have a weaker sex drive; if makes you less horny in case you have a stronger sex drive.
It focuses more on love, the romance, the support, and the relationship instead of focusing on superficial things like controlling sexual behaviour.
I'd also like to add that sleeping with someone is very different from 'seeing' them. You have sex with someone if you sleep with them, you go on a romantic date with a dinner, a film, and a nice walk along the coast at night with flowers and stuff if you're seeing them. With the latter, you may not even have sex with that person. They are very different and in an open relationship you should be able to negotiate where the boundaries are; even regarding the sleeping bit you can say 'no kissing' or 'no repeats', or anything like that.
Though personally I don't think it makes the relationship more 'exciting'. If you have sex with someone else together as a couple then perhaps; if he's referring to having more things to talk about/share, then perhaps. But it per se I don't think is 'exciting'.
Restricting sexual behaviour is no different from restricting someone from going out with other friends. It's just controlling, a lack of confidence in yourself (unless you're just controlling), or a lack of faith in the relationship. If you care about happiness and love, you shouldn't fear that your partner is going to leave you for someone just because s/he slept with someone else. If anything, there's less chance that your partner would leave you for someone else (doesn't mean you can't break up for other reasons) since being with you doesn't restrict their options in sex. If you really want to discourage sex with anyone else but you, you should have sex with him more and spice up the sex life so they won't feel the need to sleep with anyone else.