I think the crucial thing to consider here is the baby. How old is he/she? You might not be too pleased to discover that often babies make a lot of noise at night and a couple with their first infant will not be the most laid back of housemates. It'll be the most testing time for their relationship - frankly I'm quite surprised they're considering moving in with another couple! You'll all be laying yourselves open to all kinds of problems, from them getting jealous of the things you do that they can't anymore, to you getting annoyed at their lives revolving solely around the child, to them asking you to babysit when you have better/more important things to do, to you all getting annoyed with each other when you've had too little sleep. If you want to move in with your boyfriend I'd think carefully about whether it's a good idea to share a house with someone else. Would it not be possible just to move in as the two of you? Apart from this particular couple with their baby, there's the other problems people have mentioned in this thread, plus there is always the possibility you two will split up and I know that causes incredible problems in shared houses, with people feeling they have to take sides, awkwardness, who moves out etc etc.