The Gemara discusses a case where one is mekadesh a woman by force, until she consents to the marriage. Mar Bar Rav Ashi holds that although a sale of this type would be binding, by marriage it would not be, because chazal uproot such a marriage. The gemara goes through the whole routine that it does in many other places (hesubos, yevamos, gittin) and says that we understand the mechanics of how they can uproot the kiddushin if it were done with money, but if the kiddushin was achieved through bi'ah, then what. To which the gemara responds that the chachamim turn the bi'ah into a bi'as ze'nus.
Normally, when the gemara goes through this, it prefaces by saying - כל דמקדש אדעתא דרבנן מקדש. Here the gemara doesn't do that. The Rashbam explains that the gemara is relying on that here as well. Therefore, the q and a of the gemara focuses on implementing this ability to uproot the kiddushin. By kesef they have the ability to remove the kesef kiddushin by making it hefker thereby uprooting the kiddushin, but by bi'ah what can they do with the bi'ah. They can't uproot the act of bi'ah? The gemara responds that they can turn the bi'ah into bi'as zenus.
However, Tosafos holds that the gemara intentionally deletes the concept of כל דמקדש אדעתא דרבנן מקדש, because that only applies when the original kiddushin was done according to the standards of the rabbonon. It doesn't apply here where the entire marriage was forced. Rather, chazal are using the general power that they have to uproot torah principles so long as it is passive. Based on this approach there is no reason that this power should be more applicable to kesef, than to bi'ah. Therefore, Tosafos explains that the issue in the gemara is not whether they "can", rather whether they "would". By kesef they are not causing any aveira by uprooting the kesef kiddushin and making it hefker, but by bi'ah the gemara questions whether the rabbonon would actually do such a thing. Would they turn the bi'ah into an aveirah in order to uproot the kiddushin, to which the gemara ultimately says, yes they would.
Why is the gemara content with uprooting the original kesef kiddushin. Presumably they have had relations throughout the marriage, any by uprooting the kiddushin it would turn the bi'ah of the past # of years into a bi'as ze'nus. Why is the gemara not troubled about that, just as it is about the bi'as kiddushin? According to the Rashbam it makes sense. The gemara is only concerned about what power the rabbonon are using, but is not concerned with turning bi'ah into ze'nus. But, according to Tosafos, why is it תינח דקדיש בכספא - what about all the bi'ah that they had thinking that the marriage was actually binding (unless you say that they would realize immediately that the marriage wasn't binding - but the question can still be asked in the other cases such as kesubos and gittin where something that happens 25 years into the marriage can cause the chachamim to uproot the marriage from the start)? It seems that the gemara understood that chazal weren't concerned with doing something that would "cause" the bi'ah to be bi'as ze'nus, so long as they are not actively turning the bi'ah into bi'as ze'nus. The only concern of the gemara is whether the rabbonon would actively turn the bi'ah into a bi'as zenus, which only applies when the kiddushin was done through bi'ah.