Well it’s hugely different. Firstly she’d need to have a number of one night stands either each month around the time of ovulation or every 2-3 days and would have to find someone willing to not use protection each time. She would be opened up to the risk of STDs (including some which may be life threatening and/ or pass on to her child) or even potential violence or sexual violence at the hands of these men she does not know. The man she conceived with would, if they found out about the child, have the possibility of asserting parental rights. Plus she’d potentially be having sex she did not want and why should any woman have to do that?
By contrast with IUI she’d be safely using a screened donor in a controlled medical setting, fully consenting to the process and at a far far lower risk to her health. Her child could, if a UK donor was used, contact them at 18 but the donor would not be able to come into her child’s life before that and try and take an active parenting role.
I’m not a single parent by choice and I’m not 100% sure how I feel about donor conception as far as the rights of the child go but I can fully understand why women don’t want to just have lots of unprotected sex with strange men.