Masturbating

My SO masturbates pretty often just about everyday. I'll hear him in the bathroom and while he's taking a shower moaning. I've confronted him about it nd he gets really defensive nd tells me it's not going on. We have sex basically every day sometimes 2 or 3 times a day. I don't understand why he feels the need to do it it makes me feel like I don't please him sexually nd like I'm not good enough in that department? He knows this nd continues to do it. I don't know what to do ?