If you were to perform a sexual act before and / or after a workout, does that interfere with muscle growth?

No . Performing prolonged sexual acts before a work out might decrease your ability to maximize your effort during your work out, but even that would be unlikely. If you feel up to sex right after your woriout, you may not have pushed yourself very hard in your workout. But, if you are truly worried that good sex will interfere with a good work out, it might be time to re evaluate your priorities!