I love @thursdaysgeek's point about keep the focus on the work.
If you talk to the boss - I absolutely recommend that the discussion center around issues and blockers in your work because your coworker's aren't there to do their jobs in a way that impacts you. In particular, it's good to have metrics for what's going on. Being a crazy spy and checking on their outages down to the minute is a waste of your time. But if you look for a coworker at 2:00 and look every half hour because the issue is urgent and don't find them until 4:30 - it's safe to say they were unavailable for 2.5 hours. Similarly if you write an email an have to wait 3 days for a response. Keep a light weight note of how much and when this is happening - generally it's not a problem when it's rare, it's the frequency of such behavior.
Public demonstrations of affection (PDAs) and/or lewd behavior in the office is another story. If the two are putting on a display of behavior that is better taken up in private in an office environment, it's OK to complain that this is NOT OK. That includes quickie's in the stairwell ("we don't want to see you naked" seems like a fairly safe office rule for most workplaces), and also any really long lingering kisses and heavy petting. The team may have to get over quick pecks on the cheek or hand holding at lunch - but this sounds quite a bit beyond that.
For inappropriate sexual behavior in the office, this falls into the sexual harassment window. The general approach is to take the pair aside and ask them to stop the specifically inappropriate behavior. If they are normal people, the fact that you are blandly stating the problem and asking calmly for a fix may just be enough to embarass them into stopping. If it isn't, take it your manager or HR.