Once it becomes resistant to antibiotics, MGen could leave up to 4,800 women infertile each year in the United Kingdom, according to the association.
The British Association of Sexual Health and HIV (BASHH) believes 3,000 women a year could lose the ability to have children if the STI becomes untreatable.
It can also be treated by an antibiotic called macrolides, but the guidelines warned that MG is becoming increasingly resistant to it.
You can get it by having unprotected sex with someone who has it. Condoms can prevent this spread. It's estimated that an MG infection exists in 1 to 2 percent of the population at the moment, with rates being slightly higher among women than men. "So people need to take precautions".
Although tests for MG have been developed they are not now available at all clinics.
MG is on the rise globally.
One particular macrolide antibiotic, azithromycin, still works in most cases however.
Peter Greenhouse, a sexual consultant in Bristol and BASHH member, advised that people be more cautious by using condoms.
"It's about time the public learned about Mycoplasma genitalium", he said.
"Our guidelines recommend that patients with symptoms are correctly diagnosed using an accurate MG test, treated correctly then followed up to make sure they are cured".
New guidelines from the British Association of Sexual Health and HIV say doctors should use tests that specifically look for the MG infection for anyone with certain symptoms, including bleeding, discharge and genital inflammation.
Almost half of 16 to 24-year-olds admit they have had sex with a new partner without using a condom, a Public Health England report said in December.
The most recent figures from Public Health England show that diagnoses of syphilis are at their highest level for almost 70 years, with 7,137 cases in 2017, a 20 per cent rise on the previous year, and more than twice that recorded in 2012.
"The new BASHH guideline on MG is a positive step forward to improving testing and diagnosis", said Helen Fifer, a consultant microbiologist at Public Health England.