My new girlfriend has only dated well built guys? How do I get a better upper body?

My new girlfriend has only dated well built guys in the past (army build) and I feel pretty inaduate compared to them, I am skinny and my upper body is weak, my lower are better built most. I'm concerned when we have sex she won't be super into it as my upper buddy isn't great. I'd like to be better for her so any advice on getting my upper body looking good without going to the gym?
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For the record I have weights at home
My new girlfriend has only dated well built guys? How do I get a better upper body?
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