incompetence or malice

girlfriend inserting large dildo or shampoo bottle in her vagina before sex so that her vagina becomes wider so that boyfriend can't feel anything when they have sex, also it makes boyfriend suspect that girlfriend has sex with other guy with larger penis or with many guys 

because girlfriend's vagina is too wide, boyfriend becomes sexually frustrated from not feeling anything when he has vaginal sex with his girlfriend, so boyfriend wants anal sex with his girlfriend because anus is usually more tight than vagina, so that boyfriend will be able to feel something when he has sex with his girlfriend, girlfriend refuses anal sex with her boyfriend to keep her boyfriend sexually frustrated, 

even though most women would enjoy anal sex if they tried it 

girlfriend giving boyfriend a blowjob, but does not let boyfriend ejaculate in girlfriend's mouth, this prevents boyfriend's from feeling pleasure from his orgasm which is very frustrating, 

even though most women would feel it's sexually exciting when a man ejaculates in their mouth if they tried it 

do keep in mind that most young women are sexually naive and easily influenced by their friends 

incompetence may be a factor, could be that girlfriends don't understand that blowjob were boyfriend not ejaculating in girlfriend's mouth removes pleasure from boyfriend's orgasm

it is not unlikely that intelligence services, security services, militaries and police and their proxies and networks teach women such behaviour