When A Man Loves A WomanOnce you become friends with the Sim, which occurs when you have a Daily relationship score of fifty or more, you can start working with the romantic interactions. You'll need to start with something simple, like a Kiss: Peck, so that your Sims get a crush on each other; from then on, you can start attempting progressively more daring interactions.
(It's worth noting here that all of the Sims in the game are bisexual, in that you can attempt a romantic interaction between any same-sex couple and have the same chances of it succeeding as if it were an opposite-sex couple. You cannot, however, initiate any romantic interactions between an Adult or Elder and anyone Teen or younger; Teens can only have romances with other Teens.)
As you proceed further along the relationship bar, you'll be able to try the aggressive actions. It's difficult to fail with timid interactions like Kiss: Peck, or Hug: Romantic, but you'll need to have a Daily score of at least 65 if you wish to succeed with things like Kiss: Tender or Flirt: Hit On. The Mood of your target is of primary importance in these kinds of interactions, so it's best if you save the riskiest interactions until you've fed the Sim and allowed him or her to take a trip to the bathroom, if they seem to desire one. Unfortunately, you can't tell what the Mood of a visitor to your household will be at any given time, so you might want to use a simple interaction like Kiss: Peck to gauge their receptiveness to romance. If they reject your relatively safe interaction, you won't have much of a chance to get a riskier interaction through.
WoohoosWhen you've managed to get your hooks deep into another Sim, you can attempt to perform The Act with your lover.
A Woohoo is the Sim terminology for, uh, tender loving care. You'll need to have a bed or a hot tub in order to Woohoo, and your Sims will need a bit of privacy as well. (If you're of an exhibitionist bent, you can head out to a Community Lot and try to lure another Sim into a dressing booth with you for some public Woohooing.)
In order to Woohoo, you'll need to maneuver your Sim into the hot tub, or have her Relax on a bed. After you're in position, find your love target and select Ask To Join from the radial menu. When they've arrived, you can select Woohoo from the radial menu on the bed, or go from Cuddling to Woohoo in a hot tub. Woohooing isn't easy, though; in most cases, you'll have to have a Lifetime relationship score of at least 65, and the other Sim will have to be in a good mood. Otherwise, if you're up to a Daily relationship of 90 or so, you should be able to get a Woohoo off regardless of any other factors.
In addition to Woohooing, you can also select Try For Baby from the menu. This has the same chances of succeeding as a Woohoo, but will increase the likelihood that a baby will result from your congress. This is obviously only available for heterosexual couples; same-sex couples that want a baby will have to save up a few thousand Simoleans and call for an adoption.
If you manage to pull off a Woohoo, you should be forewarned that any other Sims within eyesight will have strong reactions to the act, especially if they have a crush or are in love with either of the Sims doing the deed. If this is the case, you can expect some slap-fests to occur after you both get out of bed or the hot tub, with some concurrently bad memories for everyone involved. In point of fact, your various loves can find out about your indiscretions even if they don't witness it directly; Sims who do witness a Woohoo are often liable to Gossip about it to other Sims, especially if they're Grouchy. If your significant others hear about a Woohoo through the proverbial grapevine, they'll react just as strongly to the news as if they had seen it first-hand. Your best bet is to do your best to ensure that no other Sims are able to watch you while you Woohoo; if you have to, enter the Build mode and move any doorways leading to the bed or hot tub to another area of the house.
ProposalsThere are a number of different proposals in Sims 2, from the relatively innocuous, to the expressions of utmost commitment. If you have a Proposal available between yourself and other Sim, the option will appear on the radial menu; if it isn't there, then you'll need to work on your Daily and Lifetime scores a bit more before you can try for one, and be sure that the target Sim is in a good Mood! (Again, note that having a Nice score of 10 will make many of these almost automatic if you have the required Lifetime relationship score.)
Teens have the ability to initiate a few Proposals, such as Sleep Over or Sneak Out, but the most important one is Go Steady. This is a Want for many young Sims, regardless of their Aspirations. (Although Romantic Sims may have it as a Fear, instead.) It can be difficult to pull off, though, unless your target's Mood is above three-quarters full or your Sim is extremely Nice.
The first important adult proposal is Move In. Be aware that, if this proposal works, the target Sim will join the household of the proposing Sim. This can make for some crowded homes, especially if no one moves out at the same time. If you're ready for the commitment, though, be sure that your Lifetime is above 60 and that your Daily score is above 70 before you try for this.
The next rung up the ladder is an Engagement. This doesn't have too many gameplay effects; the two Sims will act much like any other pair of lovers. If you want to try to get engaged, you'll want to have both your Lifetime and Daily scores above 75.
If you do get engaged, though, you can try to achieve the ne plus ultra of Sims interactions: a Marriage. (Sims of the same sex will have to settle for a Joining; no doubt there are enterprising modders working on a Civil Union patch as we type this.) Marriage requires a Lifetime score of at least 75, making this the culmination of a long sequence of socializations; most Sims will in fact require a Lifetime and Daily score of more than 80. A successful Marriage comes with all the baggage of a Move In, but you do gain a bit of cash, which can be helpful, especially if the Sim you're marrying is especially wealthy.