When loving parents make decisions concerning their teenage children, they put very small, if any, weight on the physical pleasure of sex as isolated from other things. For instance, that some course of action is likely to increase the number of times that the child experiences the physical pleasure of sex counts very little in favor of the course of action—it may even count against it. Parents are either mistaken in this weighting or not.
We would expect parents to be more reliable in weighing the values of pleasures of activities that they themselves find pleasurable. But typical adults do find sex pleasurable, and presumably that includes the parents (and may even help explain why they are parents). Indeed, typical adults find sex physically as pleasant as teenagers do, or more so due to greater experience. Moreover, I think we would expect parents if anything to be better judges of the values of outcomes in the case of their children than in their own case, at least when the outcomes are ones that the parents are familiar with and find pleasant or unpleasant in much the way that the children do. One is likely to be more clearheaded when one is making decisions for someone else.
But if this is right, then parents are probably right when they put very low weight on the physical pleasure of sex in the case of their children. Thus, probably, the physical pleasure of sex has very low value in the case of children, at least in itself. But since this physical pleasure is basically the same in adults (perhaps somewhat greater due to experience, but likely not an order of magnitude greater, at least in males), it follows that the physical pleasure of sex in and of itself, isolated from other considerations, has very low value in general. (Of course, the pleasure as combined with other goods may have significant value.)