its a Handy world

How do you feel if you interrupt your partner while masturbating?
 Betrayed? Offended because they didn't play with you? Turned on because they decided to take pleasure in hand?

There are many reasons people masturbate during the course of a relationship. It may be as simple as they're horny when their partner is at work or busy with something else. They may feel the need to have an orgasm, but may not feel they have the time or energy to pay attention to their partner, and therefore don't want to initiate sex. Their partner may be tired, sick, asleep, having their period,pregnant or stressed, and not interested in having sex. Others have said masturbation helps them fall asleep easier, or it simply feels good, so they do it. 

Masturbation is not a betrayal , its a normal part of a sexual relationship with your own body.The emotional component people feel when stumbling upon or accidentally interrupting a partner when they masturbate is unusual because the feelings of betrayal and hurt are common.

  A person can have a healthy, satisfying sex life and continue to masturbate. The fact that your partner masturbates does not mean that they aren't satisfied with your lovemaking capabilities. It also does not, as some people believe, indicate that your partner is having (or considering having) an affair. If you still have doubts, the answer again lies in your partner. To find out why they specifically masturbate, you'll have to ask them. This is an uncomfortable prospect to anyone who may have underlying communication issues within their relationship.

However a relationship does not grow stronger without honest and open communication.

Masturbation is the first form of sexual interaction a person has, its elemental, and doesn't cease simply due to relational status The issue of  Masturbation brings along with it the 

 discovery of  finding them using pornography whether from the internet or magazines or sex toys.

 I sincerely suspect that this is where the real insecurities lie, while most well adjusted men and women realize porn and sex toys don't replace face to face human sexual interaction , but since we are all programmed from youth forward with insecurities this is an emotionally charged issue.

Honest communication is the best reaction to finding your partner engaging in self satisfying behaviors. But please be polite, and let them finish first.