I find the definition of a 'bdsm relationship' too general, what really is a bdsm d/s relationship in the definitive terms? 'Sex' is a subject that needs further clarification as well, because I'm taking it to be defined as 'coitus' in this topic.
The reason I ask this is because in every relationship, there is an element of d/s and individuals that don't 'actively' practice bdsm: in the form of 'parties', clubs, equipment or 'standard' definitions/roles. These individuals might not effectively realise or even define the bdsm element as 'bdsm' in their relationships but it's there. It can be there in the way that one person addresses the other, the authority one takes over the other person in bed or out.
Sex, sexual activity or the erotic element of a relationship (say between two people or more etc) as it is in 'conventional' non bdsm relationships -those that don't 'overtly' display bdsm roles in the way we are 'accustomed' to them - is not altogether a totally different concept. Sure there are individuals that might be 'sexually' satisfied (arousal, climax, resolution) by engaging in acts that don't necessarily conclude in any type of physical 'intercourse', but even that is sexual as well, to them it's sexual. A person might not have the desire to 'touch' another person, and be stimulated to orgasm just by being verbally 'humiliated' etc - non active or sexual participation, that is what they find erotic. Others get that mental satisfaction, not so much sexual satisfaction by bending another to their will etc, this can be sexual or it might not be but this can also take place in daily life as well where we don't observe the person being sexually aroused physically, but they might be aroused mentally - and even that can't be defined as 'non participation', as there 'is' participation although it's subdued or subtle. Having a slave or 'training' a slave, the procedure of bending another person to your own will, does give satisfaction to some, and there is an erotic element in that, the 'pleasure' principle of arousal is traversed. Sure it isn't 'physically sexual' in terms of people sexually interacting, but it's just another dimension, otherwise people wouldn't offer themselves up as 'potential' slaves or dominant people would not 'train' them.

Sex (in terms of coitus) doesn't require love in non -'bdsm' relationships for some, for others it's essential. Love is not essential for sex to take place in other words, for some it does. For others, for any type of bdsm to take place, love is 'essential' as 'love' is associated with 'trust' , but this is not essential either. Trust is an important concept in all types of relationships, just because a relationship features d/s or whatever other roles, doesn't mean that just becuase it is 'bdsm' related it 'should' have a higher 'trust' priority. Trust is trust.
In other words, bdsm relationships don't largely differ to any other types of relationships. Initially I thought that there were other 'factors' within them that set them apart from everyday conventional (sexual or not) relationships, but now I see that the dynamics are pretty much the same - a person can lose control in coitus or sex, just as they can when they restrain someone in bdsm. Rape play does occur in non bdsm relationships, just as spanking does, the fact that people might not openly categorise themselves as being a 'slave' ,'sub',master or openly attend bdsm parties etc, does not mean that they are poles apart from those 'in the scene'.

For me submission means the very definition of the word, it can be a sexual preference or a psychological state at a particular time. For others it is a full time state of mind which can be observed in the way they interrelate with the general circle of people around them. The same for dominance. I've experienced men that take a dominant 'role' in terms of role play only, during coitus but outside of this they are largely neutral in the everyday, whereas I have also experienced men that are just dominant in terms of their personality trait. I'm not submissive 'by nature', in that I will 'give' in my everyday, and I can appreciate that others are. For me submission is largely erotic, but not something which I will carry through in my everyday life 24/7, it all relates to giving 'sex' or erotic play more nuances, focusing on a specific end goal, increasing the stimulus somewhat. That's how I see bdsm in general, an increase of stimulus being required in order to reach that end goal of sexual satisfaction - sexual doesn't mean just physical release, but the mental interplay. If the mind doesn't 'engage', then our body won't respond in all areas of sex whether it's bdsm, or 'standard' (vanilla is just too 'categorical' a term for me to use).

Sometimes I feel that there is too much categorisation in bdsm, some stereotyping as well as way too many 'definitions', making it like a 'sub' category relating to 'relationships' - the bdsm vs 'vanilla' relationship for example. Just because some couples or individuals don't prefer to have a red raw ass, or want to practice suspension or other acts, doesn't necessarily mean that they don't incorporate bdsm in their daily life. I've seen men that do the whole 'spanking', costumed 'domination' bit, and that's fine, then again I've seen domination take place amongst some people I've known, in pure verbal and mental form amongst people that aren't 'effectively' in the scene itself - by scene I mean those that know all the 'terminology', that have bdsm 'aids' or go to dungeons etc etc.

Relationships are just that to me, relationships. That is the fundamental term. The foundation of any relationship can vary. Sexual relationships are just that as well, because every individual might have varying stimuli they find sexually arousing, but the end point or end goal is the same - sexual gratification. If love features, it's a bonus, if not, then that's fine as well, everyone is individual, and in relation to 'romantic' love or any type of love, it's more a time and place 'thing' - meeting that person at a particular time when one's frame of mind is ready to make that extra commitment.