My advice get yourself your own girlfriend.
If down the line she decides to date your husband also down the line organicly then awesome.
People are not toys to be shared.
40 yo straight female
Married in the eyes of the government to Butch since 2001...
Murf my monogamous second husband has been with me since May of 2012.
In a V relationship with an average 60/40 split of time. Only due to Murf's and Butch's crappy work schedules.