Scot-Pep says it has been barred from today's meeting despite having been previously invited to give input into the debate.
The charity opposes a proposal by Labour's Rhoda Grant to make it an offence to pay for sex, which the MSP says will reduce the demand for prostitution.
Laws already apply in Scotland to soliciting and loitering, such as kerb crawling. An attempt to fast-track The Purchase Of Sex Bill was rejected at Holyrood in 2012 and it was put to consultation.
Ms Grant says the meeting is for those who support the principles of the proposed legislation and she is happy to meet the group separately.
Scot-Pep says criminalising those who pay for sex hampers sex workers' ability to screen clients for their own safety. It says the introduction of similar legislation in Sweden has been associated with "significant additional levels of violence" against sex workers.
It fears the proposed Bill will have a better chance of succeeding without the input of the late Margo MacDonald MSP, who campaigned for sex workers' rights.
Scot-Pep board member Neil McCulloch said: "Disastrous as this legislation would prove in Scotland, we nevertheless sought to play a constructive role in discussions and are shocked to have been excluded entirely.
"Would you have a meeting to discuss whether to legislate supposedly to help any other marginalised group and yet explicitly bar them and their representatives from the discussion?
"It can't be right for a politician to try to work in this way without even wanting to hear the voices of the people most directly affected."
Ms Grant said: "The planned meeting is for those who support the principles of criminalising the purchase of sex and as Scot-Pep has publicly stated its opposition to this its inclusion would not be appropriate at this time. I would be happy to meet its officials separately."