I've been room-mates with married couples on several occasions, without problems. They don't engage in particularly intimate behaviour in the common areas (sitting cuddled up on the couch watching TV or exchanging a simple kiss/hug is fine, making out is not). If and when they are engaging in activities in their own room, they are reasonably quiet about it. And they keep arguments away from the room-mates.
For a short term visit, a couple can suck it up and keep things PG for a few days or weeks. Same with sharing a hotel room with another couple.
In an emergency, accommodations can be made. If someone's had to move in suddenly, and doesn't have their own room, for example, quietly arranging to leave the couple alone in the house for a few hours every week (with them knowing what the schedule is), so they can have some privacy even if they don't have their own room. From the point of view of the visitors, they can do the same - make sure the're out of the house for a defined period on a regular basis to give the regular owners/tenants some of their own space back.
In the case of a couple moving in with family for an indefinite time, some arrangement needs to be made so that the couple gets enough privacy to engage in couply things - serious discussions without an audience, snuggling and kissing, Scrabble - on a reasonably regular basis. Doubling up kids so the couple has a separate room is one option, turning a den into a bedroom is another, vacating the apartment to leave them alone is a third. Some effort has to be made to accommodate the couple. Something like sleeping in the living room between the hours of midnight (when the last adult turns of the TV and goes to bed) and six am (when the first kid gets up to watch TV) is not a reasonable solution. ON the other hand, the couple taken in needs to adjust their own behaviour, recognizing that they are sharing living accommodations.