it really depends on what kind of people are you, either responsible or not. having s*x, i think is not irresponsible but although our culture tends us to be one, my point is as long as you are aware of the consequence and ready and capable to take the responsibility then i think theres no problem with that.
the only thing, i think is bad when having s*x is not taking into considerations the effects of the said actions, like having a baby and doing the wrong action to compensate the misdoings.
also, s*x gives spices to the relationship. without it, i think it would be boring when your relationship have gone over a year or longer.