Is it ok to start looking else where for sex when your in a relationship?

[only me]

My current partner has lost all interest in sex since having our second child. It is driving me mad I am lucky to get it once a month (and thats with begging) and when i do get it its just a quickie its like im doing a corpse (complete shut down on her part) Its alway make it quick not touching and kissing. I have tried telling her and it has no response, so lately i have tried to explain how much it is effecting me, to the point where i have told her if it doesn't change soon i might go off and get it else where!! Is that really that wrong. It would destroy me if she did it, so am i being out off order? and any suggestion as to what i can do to get her back into sex? I dont want to go off with any one else but this is getting silly now (we used to have such a good sex life). she has said to me that she wouldn't care if i got it else where if it meant i stopped going on at her. I would happily wait once a month to do it if it was her i was doing not her in a shut down state ( completely blank as if she's asleep)