First of all, it's not you!!
Plenty of husbands manage to get through post-birth/less less sex phases without resorting to other sexual "pursuits".
I'm sure he could give you all kinds of gas-lighty justifications and excuses, but none of these will mean much of anything because the bottom line is that if this makes you feel icky, grossed out, or uncomfortable, that is a complete reason to question if this is a relationship you can realistically continue with.
You might just be incompatible in terms of what kind of sexual promiscuity/behaviours you are each comfortable with, within your relationship. You will have decide if you are happy to have a relationship with someone who engages in these behaviours, or if you are not.
The second issue is that he has also been deceptive, because he knows you wouldn't like it. So that is a separate issue you will need to consider. Are you comfortable building your life with someone who is comfortable being dishonest with you?
Checking someone's phone isn't ideal, but if you have a feeling something is going on, and it turns out it was going on, I'd say the decision to check his phone was justfied and the "lack of trust" is a non-issue compared to when trust has been broken. (Unless there was a specific agreement between you that you are each allowed to engage in porn and massages without informing the other.)
The implicit understanding in relationships would be to assume disclosure should be made about any sexual interests that might affect or upset your partner, and one should respect their partner enough to allow them to choose if they are comfortable with that or not.