Well, you could go to a public lot, either put a photobooth or a hot tub inside there, get a sim with a high relationship with the one who wants to woo hoo. For Hot Tub: Enter then both in the hot tub, make them cuddle, and make them woo hoo. If it's the first time they've done it public a cut scene will come up.
Booth: Make one sim enter, and either call over the other one.. or if they're both selectable, choose the other one and click it and make that one enter. Then when they're both in a sitting down, click on the booth again and it should say "Romantic , Silly , Woo Hoo." (It may have another picture taking option but I don't know. ._.)
Hope that helps?