Women using unregulated websites to find a sperm donor have been harassed and sent photos of miscarriages, the BBC has found.
There are strict criteria for NHS artificial insemination and many women say private treatment is too expensive, so they turn to the internet.
One woman told how a potential donor she met turned out to be married and had had a vasectomy.
A fertility expert called for unregulated sites to be shut down.
Online sperm donation is not illegal but it must be provided free, although expenses can be covered.
Some online donors offer artificial insemination (AI) and others natural insemination (NI) - effectively unprotected sex with a stranger.
"Sarah", 26, from Yorkshire, is single and said she wanted a baby without a partner.
She told BBC Inside Out the cost of private treatment was too high and going online meant the treatment was free, although she did experience problems.
She said: "There have been a few idiots that you really wouldn't want to interact with, because they do harass you or start sending you images you really don't need to see."
ouple Kirsti and Danielle, from West Yorkshire, said they had faced pressure from men online to use NI.
Kirsti said: "Some people try to put you off by sending you messages of miscarriages and stuff, like if you do it that way, this will happen.
"So they are trying to change your mind for the method, so then you have sex with them."
Sarah did meet one man online but he was not as he seemed.
"Ultimately it turned out he was married and that he'd actually had a vasectomy, so was in no way viable at all," she said.
"It made me feel like I couldn't trust people on the site.
"I think they do need to be policed by a proper organisation, not just some random person out there because you don't know if they're safe because there's no proof of who you are on there."