Interesting thread...so far everyone seems to be focused on what people will think of the sex act while noone has mentioned the common outcome of sex namely a child. No contraceptives in the Buddha's time.

It turns out that in the Sigalovada Sutta the Buddha recommends that both husbands and wives should be faithful to each other. So, although not explicitly covered by the Third Precept, unfaithfulness and having children outside a committed relationship are regarded as unwholesome actions.

It turns out that in the Sigalovada Sutta the Buddha recommends that both husbands and wives should be faithful to each other. So, although not explicitly covered by the Third Precept, unfaithfulness and having children outside a committed relationship are regarded as unwholesome actions.

Did the Buddha cover anything about sexual relations outside of marriage?

No, nothing specifically about pre-marital sex.