What is it about stockings and heels that turn a man on? Also, does it matter if you know the girl or not?
Well, where to start??!!
I will not try to speak for men in general, but for me the association (of shapely legs, accentuated by stockings) with erotic activity is part of the answer. Where does that association in my mind come from? A mixture of cultural norms in my society and personal experience.
From puberty onwards, I'm used to seeing women who are sexually provocative, or just plain flirty, often wearing stockings and heels. They also wear plenty of other things, and it's not an exclusive requirement, of course, but merely one of many optional signals that, to me, conveys the notion of a women wishing to display confidence, who may be wanting to stimulate or attract males, or even to dominate the agenda to some extent.
The personal experience aspect includes that fact that stockings are nice and smooth and make it easy and enjoyable to caress a woman's legs. Doing the same with bare flesh is also very enjoyable, but is different, so variety and contrast come into play here.
As for high heels, another factor is that they elevate the woman, even if it's by only a few extra inches that's enough to make her slightly more prominent amongst others standing nearby than otherwise she would be. That helps mark her out in way which, to me, is attractive and exciting. The idea that the woman is also having to balance with care on a slightly less than fully-stable platform also makes it a little risky, in a sense. The teetering leads to dramatic tension - perhaps the woman feels totally secure herself, but it doesn't look that way to me! Part of it is, maybe, a bit like some people feel about simple bondage - the woman is constrained by what she is wearing and has to stand in a particular way (often with pronounced posture that highlights other physical feminine characteristics) and so is less free than were she to be wearing flat shoes.
The fragility of sheer stockings also creates a sense of excitement and also means that they are often brand new. It's always the case for my wife when we go to a special occasion (like a wedding or big birthday party) and that extra trouble and expense to look good sets up good vibes for the occasion itself and what we will do together afterwards.
Because of the constraints and formality that both high heel shoes and stockings impart, when - eventually - they are removed, it brings a release of tension that works well in engendering play between consenting adults. The freedom then obtained encourages sexual expression to great effect.
For me, a garter belt is a great addition and far preferable to thigh-highs or other 'stay-ups'. The garters themselves, and the tight belt around the waist, all add significantly to the beauty of the overall sight.
Does it matter if I know the woman or not? Not especially, but it's more enjoyable if I do know the person because I can relate to her and my imagination can more easily generate scenarios that are more realistic than if I'm looking at a stranger.
Hope that's helpful feedback for you, Charlotte, but not sure if it will let you 'get it' / understand your bf's point of view and his reaction to you wearing stockings and heels - do let me know!