we were too caught up in the sexy

Difficult to draw too many conclusions based on a trailer - we don't know the sequence of events . . .

Although it does seem to be a scene of showering after some event in a state of relief . . . You'd (well I would be, for sure) probably find sex the last thing on your mind if you knew one of those things was on your tail.  However, if you'd just escaped a near death experience the need to reproduce could well be more 'pressing' shall we say ;) 

My guess is they thought the danger had gone but . . .

Exactly. They thought they were safe, but some time had passed since the event, given the size of the Xeno. They didn't know about the Captain. It's really fascinating what that implies.

Indeed . . . but maybe this one didn't come from the Captain?!? ;)

That's a terrifying thought. 

They could really be trying to throw us off if that is a dream sequence.

Well played! Though that feels unlikely

I feel like this could be toward the end of the film. These are the last two survivors and they think they are safe and away and relaxing from the stress of the horror, and then.....so they aren't IGNORANT of the threat, they just think it's passed and they think they've gotten away 

That's a great gotcha moment! Talk about ending on a Downer!

If it's going to end there they need to do something that no one will expect- something that will make the audience squirm in their chairs. Something gruesome  that may actually offend. Bring it, Mr Scott. Anything else would be an eye rolling shower/horror scene we have all senn before.

There is that. That's a huge possibility for Eww. Alan Dean Foster's Alien 3 novel did include Xeno rape in order to produce a queen.