TL - DR
If body surfaces of any type, and fluids of any type, from 2 or more people come into contact OR ARE TOUCHED BY THE SAME SIDE OF AN OBJECT, than those individuals are conventionally considered fluid-bonded, which has STI/Fluid-Borne Illness implications. There are lots of ways to achieve this, sometimes even accidental.
Basically, if Person A uses a toy, and Person B uses that toy, they are fluid bonded - it's assumed fluid transfers with the toy. If A and B have some kinky unprotected fun....they are fluid bonded. Condoms stop fluid bonding when used correctly. Dental dams stop fluid bonding for oral when used correctly.
This is important, specifically in the prevention of STIs, and other fluid-pathway pathogens - so named because, well, they can transmit through the exchange of such fluids. For some, it's also a significant expression of trust and acceptance. Historically, for example, the consummation of a marriage (let's not debate this here, I only acknowledge its significance in history).
MORE DETAILED THOUGHTS
A fluid bond is an agreement between n partners (n>1) to share equipment/intimate contact that in some way touches significant bodily fluids.
If I cum inside of Playtime, without a non-permeable barrier in place (read: a condom) - we are fluid bonded (we are).....it's about sharing fluids across areas that fluids can pass - one, or two directionally. Anything internal can pass fluid. Injuries pass fluid. Vagina, rectum, penis, urethra, mouth, etc. Fluids are defined fairly simply.... (that being said - some exclude sweat and other EXTERNAL secretions)
The reality is that, in most cases, sharing 'kinky' equipment leads to fluid bonding. If Playtime uses a dildo, then I use it - we are fluid bonded - any surface that the two share can cause bonding.
EVEN MORE DETAILED INFO:
In the professional world, we mitigate the spread of STI/fluid borne illness through sterilization or disposability. Sterilization is a process that is rather difficult to achieve correctly as a consumer, and for which a great deal of mis-information exists...Boiling something for 5 seconds doesn't sterilize it. Washing your stuff off in a sink doesn't sterilize it.
For some, even this doesn't mitigate fluid bonding....
The reality is, even the modern medical community has decided that disposability is, at very least (more often than not), cheaper than sterilization done right. Let's say you have a hospital enema kit - it's cheaper to throw it away and make a new one than to try to sterilize (Ethylene Oxide...plus labor....isn't cheap) a used one, and unless Walmart starts selling EO Packets/pouches (or other methods of sterilizing plastic - believe me - none of which are easy), and we train the entire world on cleaning/etc.....sterilization of most material is out-of-bounds for most. Metal is somewhat the exception to this rule, since it survives significant heating, for extended periods of time, without excessive deformation.... autoclaves are a good example of this ..... but even then, that requires time, training, and proper execution.....
But even then, the value of the object being sterilized must be significant to warrant even bothering.....
That said - most of us medfet enthusiasts can't exactly simply order up from a local medical catalogue. But everything is a tradeoff - as long as everyone involved is aware of what is being traded, and what the real risks are....stay kinky.....