I think that a long-term and serious relationship should involve at least regular sex in order to solidify a greater sense of physical intimacy with both partners - of course, this goes without saying that every other mutual act (dining, hanging out, romantic gestures of a non-erotic nature, etc.) would also be more or less significant in defining how a relationship works amongst different couples everywhere.
Though I myself have yet to be in a serious relationship , I can support my views from friends I know who are in gay and straight relationships as of the moment - some boast about sex nearly every other night, while others prefer to keep it strictly after marriage, yet still others say that they have yet to do the act (despite being together for over two years!) . . .