"It's insane and it's cheating," Martina says in an op-ed posted in The Sunday Times.

"I am happy to address a transgender woman in whatever form she prefers, but I would not be happy to compete against her. It would not be fair."

Martina has become passionate about the issue after an online spat with transgender cyclist Rachel McKinnon -- who won the Masters Track championship in L.A. back in October. The woman who came in 3rd place, Jennifer Wagner-Assali, said the fact McKinnon was allowed to compete against women is "not fair."

Martina initially sided with Wagner-Assali -- which led to McKinnon labeling the tennis great as "transphobic."

So, Martina says she decided to dedicate months of her life to researching the topic -- and now feels even STRONGER that trans women have an unfair advantage and shouldn't be allowed to compete against cisgender women.

"Simply reducing hormone levels -- the prescription most sports have adopted -- does not solve the problem. A man builds up muscle and bone density, as well as a greater number of oxygen-carrying red blood cells, from childhood," Martina wrote ... "Training increases the discrepancy."

"Indeed, if a male were to change gender in such a way as to eliminate any accumulated advantage, he would have to begin hormone treatment before puberty. For me, that is unthinkable."

Martina says she's up for having the tough conversations on the issue -- but feels there's a "growing tendency among transgender activists to denounce anyone who argues against them and to label them all as 'transphobes.'"