I haven't ever had this in real life, however I work as an educator for medical professionals, and in some of the cases spousal abuse questions are (unnecessarily) raised. It's tricky to diffuse them, because trying to come up with an excuse will usually result in them following it up, as the percieve 'evasiveness'.

Don't try to deny anything happened. Tell as much of the truth as you can, and put it in a positive light. Smile when you tell them everything is fine, rather than worry that you will or won't convince them People who are being abused don't (generally) smile when asked if it's happening.Since you said you enjoyed this, that shouldn't be too hard

...you and your SO were roughhousing (giggle, blush) and you didn't even realize they (the bruises) were there...

"No, mine is a very happy relationship (grin)"