Even if I were in a relationship where I could be w/my partner pretty much 24/7, I would masturbate because (1) sometimes I just want to be responsible for my own orgasms, (2) sometimes my partner may be too tired or busy to "play," (3) sometimes I may not have much time and know I can bring myself to orgasm faster than a partner can, (4) sometimes I want to use a particular fantasy while masturbating that just doesn't jibe w/partner play or that my partner wouldn't approve of, (6) sometimes my partner may want to watch me masturbate, and (5) sometimes I long for a particular feel or type of orgasm that only I can provide.

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Communication is the key.