Apart from the excellent explanations coming from Hermann, I thought I'd comment on the charge of legalism.
First, it is impossible to not appear legalistic when responding to questions like this. Unless the response is "just follow your hormones" -- which is, we all know, a wrong response, anything else would sound legalistic because it would offer rules. We all understand that sex is supposed to be a playful, happy, improvisational thing, but this thread is necessarily about rules. They are to be internalized and then the rules become simply culture. We don't consider the requirement to say please and thank-you, or hold the door for the person behind you as legalism, -- these are rules internalized as culture. Such are, or should be, all these rules about French kissing, spontaneous sex in elevators and what else was it that you asked?...
Legalism has a special meaning for a Christian. It is specifically a heresy rectified by St. Paul in Galatians, which understood Christianity as a superset and a continuation of the entire Jewish Law. Paul confronted it in two ways, first, by noticing that the ceremonial law was given to the unbaptized Jews alone and for a specific to their role in the economy of salvation reason. Circumcision simply does not apply to a Christian. Secondly, he noted that the message of Christ is that of freedom: we do things not because a law tells us to, but out of love of God and of one another.
But this: love of God the Father, who alone effects our procreation, and love of the spouse, is the core of the Catholic moral teaching on marital relations! We are against contraceptive activity and contraceptive mentality alike, because it makes our hearts cold to God. We are against marital sex that objectifies the partner because it injures the partner's dignity and cannot be love. If you reflect on the two commandments of Christ in the context of the conjugal bed, you get Catholic sexual morality, down to the last technicality of the NFP. It is, per St. Paul, the very opposite of legalism.
Well said, though I differ on the spontaneous sex in elevators . . .