The facts say the exact opposite. According CDC 2010 National Intimate Partner and Sexual Violence Survey reports on the lifetime prevalence of rape, physical violence or stalking by an intimate partner gives the following rates by sexual orientation of the partners:
61.1 Bisexual women
43.8 Lesbians
37.3 Bisexual men 
35.0 Heterosexual women
29.0 Heterosexual Men
26.0 Homosexual Men
To repeat that is violence by one partner on another. It is pretty clear which is the safest relationship for women. And for men that matter.