Did he wash off after you had sex?
I'd try not to read anything into seeing a film on your man's penis. That's not a reliable source of information in my opinion. Did you just have a baby? Congrats if you did. Do wait the appropriate amount of time though before intercourse though so your body can heal. And remember, after we have a baby, our hormones are raging. And they can cloud our thinking. Are you getting along with your partner? Are you two committed to one another? Why would you suspect cheating other than a whiteness around his penis that you noticed by glancing at it?