STTs

The following is an observation of some peculiarities in the transmission of impurity with an attempt at explanation based on their unique commonality 

It is axiomatic to the transmission of tumah that it gets downgraded one level each time it transfers*. 

For example, an avi avos will become an av and an av a rishon. 

There are two exceptions to this rule where the same level of tumah can be transferred to the recipient. They are tumas keri and tumas niddah. The former (the baal keri) is a rishon and unable to contaminate adam vkeilim and the latter (the niddah) is an av and contaminates adam vkeilim for one day. 

The above is limited to regular modes of transmission (eg magah), however, when a man and women engage is intimate relations, different rules apply. When a man cohabits with a tehorah (kdarkah with ejaculation) they both assume the status of a rishon ltumah (although semen is an av, that is not what contaminates her, as it is magah beis hasitarim which is impervious to transmission). Likewise for a bo'el niddah where the man himself becomes tamei with tumas niddah (eg he becomes an av and tamei for 7 days). 

Both keri and niddus are part of healthy reproductive function. For these tumos to maintain their respective levels (rishon for keri and av for niddus) they require cohabitation. In other words, these tumos, which are specific to reproduction, maintain their severity—despite transmission—only via biah. 

A biblical stt (sexually transmitted tumah), if you will. 

Perhaps the explanation is that as biah unifies the male and female to some degree (vhayu lbasar echad), tumos that are associated with sexual function become assumed by the other party without losing any steam in the process.

(Admittedly there are differences between how keri devolves upon a woman, namely limited to biah kdarkah that includes ejaculation, which seemingly leads to the conclusion that it contaminates her just as it does him, moreso than transmitted from him to her via biah, as then a preexisting tumas keri should transfer with any legally recognized coital act viz ha'ara'ah or shelo kdarkah, which is not the case with the baal keri, vs the niddah where any halachically valid biah act does indeed transmit niddus. This is ostensibly also reflected in that a meshameshes is exactly the same tumah‐wise as the baal keri whereas the boel niddah, despite assuming all the stringencies of tumas niddah ie av, 7 days etc, is nevertheless slightly attenuated vis a vis mishkav/moshav/merkav. I believe that despite these slight differences the overall observation remains correct)

*with respect to animate recipients; cherev harei hu k'chalal and tumas medras, which involve inanimate objects, do indeed maintain a similar level of tumah