One of the big things that comes up when a vanilla person views BDSM images is the non-consensual nature of BDSM. Often they'll make a comment along the lines of "some of these girls don't look like they're enjoying themselves," or "it looks forced."
Both of those are true to a certain degree, and both illustrate the point of BDSM. I once came across a quote online that said "bondage is purposeless without the desire to escape." Another read, "there is such a thing as consensual non-consent." To explain that a little better, the purpose of being a submissive within the BDSM culture is losing control. Submissives fit into their role because they enjoy losing control, and a part of that is being forced to do things that are less than pleasurable...because they enjoy that dynamic.
It's a contradiction for sure, but not one without comparison. Take any athlete who blocks a shot, or takes a pitch, or weathers a big hit in order to help their team, giving a part of themselves for the cause of a greater good. BDSM, specifically submission and masochism is the same. The submissive weathers the whims of the dominant because they get enjoyment out of pleasing. (And often there is a sense of accomplishment at taking an amount of pain.)
Although it looks "forced" or non-consensual, in a lot of instances it's the Dominant that is giving something to the submissive, not the other way around. Being a good Dominant involves a strong sense of respect, and with it can come an unwillingness to dole out the level of punishment that the submissive desires. After all, you want to hurt someone, not injure them. If there isn't an incentive to struggle against restraints, then there's no reason to have them. It would be like making a law that virtually no one breaks...what's the point?
Good BDSM exists with a lot of trust and a fail-safe. Partners are well aware of each others' limits and tolerances and can dole out an appropriate amount of punishment without going too far. If a boundary is crossed the submissive can use one of a number of safe-words that either dull the action or stop it entirely.
It's a very complex interaction, one that is so fluid that it's difficult to pin down in any sense. The submissive is consenting to not have consent for a period of time (within certain boundaries). They're enjoying the fact that they're enduring things that they don't enjoy. The can be made to do things that they don't want to do...but the act of being forced makes them want to do them anyways. And beneath it all, with the use of a safeword, they never truly lose control (with a good Dom). It's counter-intuitive at times, but it works.
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