Solipsis is pretty much spot on in his longer answer to the question than mine (who's ambiguity was intentional so as to carry two meanings—you either aren't on 250 µg or aren't as functional as you think. The latter is very important. If you are accustomed to feeling functional in an operational/cognitive sense in a given setting, it is in fact possible to acclimate yourself to doing quite a variety of things under the influence (like having sex—for myself, most of my partners who've indulged, and in the cases of most people who I've spoken to, the first time fucking while tripping is initially awkward in terms of position, rhythm, etc. until instinct kicks in, and this is not even to mention male difficulties, but with some tripping-sex experience "under your belt" so to speak it comes more naturally and is pretty much uniformly mind blowing at least absent some possible negative mind-set, etc.)
Problems, or increased risks, however, happen as variables multiply and change. Things might be different with a different sexual partner, you might be too fucked up and disinhibited and do something inappropriate or regrettable (I've definitely decided "fuck it, forget about the condom" more than once while under the influence of one drug or another, condoms not being very easy to apparatus manage while tripping balls; I've also been to shows and suchlike and wound up with women I never would have if not under the influence, for better or for worse.) Or if you're interrupted in flagrante, you might not have the proper reaction causing awkwardness or worse (not exactly this but something like it came rather close to getting me killed around the winter of '08.)
Or let's talk about another activity a lot of us participate in, and usually, or at least statistically, more dangerous one. That's right, we're going to talk about driving, and the rest of this post is participant–observer sociology, not harm reduction:
Lots and lots of people drive while tripping on a few hits of acid, hopefully after the peak; I'm talking about serious acid users though, usually in the setting of driving back from a show. It's often not a "designated driver" situation but a "who's best at driving tripping" situation. I've ever even had my ride ask me to throw him a few tabs to straighten him up and keep him awake for the drive home. I did, everyone lived; I've been both passenger and driver on acid more times than I can count. Highway driving feels quite easy and with good music and scenery can actually be highly enjoyable. Town and city driving is less pleasant often going on terrifying as is driving on country roads especially in the dark, although again, some people seem to be fine.
I think most of us who have been in that lifestyle and drove have done this to one degree or another. Saying that it's outright dumb shouldn't even be necessary. The very tail end of an acid trip might be about as impairing as a few beers and a joint, and, just like beers and joints, experience, tolerance and other individual factors may apply. Uniformly, though, even in people who are pretty confident in their ability to operate a motor vehicle while tripping, the risk is more or less the same: unexpected events or stimuli. While LSD does not have a lot of effect on either gross or fine motor skills (strictly speaking), and often can actually enhance visual acuity and other perceptions, it can drastically alter our cognitive coördination of these, it's speed, facility, etc. Even on the highway experienced driving trippers might be in serious, deadly trouble if the car ahead suddenly throws on the breaks; they may very well swerve appropriately but may do so into an 18 wheeler. Concentration in the "keep your eye on the road and your hand behind the wheel" sense may be fine but situational awareness overall completely another matter. You can also be rocking out to music and be followed by the police with lights and sirens blazing for quite a while before you notice (learned that one the hard way.)
So yeah, you'll meet a bunch of people who'll even start to go on about man molding with machine (even more so on bikes) and driving as an extension of their own body, etc. It sure feels like that until something and jars you out of it and this thing is just you out of it might get killed.
This goes for swimming, boating, shooting, drug dealing, any activity where potentially dangerous situations can arise quickly. Don't let your confidence or experience level or confidence in your level of experience deceive you into thinking that you can handle whatever the world throws at you in those scenarios. You can't. You can't really even handle whatever sober. Tripping your mind is going to react unpredictably, focus unpredictably, possibly have trouble recognizing and reacting to crisis.
I've been robbed at knife point while on a large dose of acid, I was basically stunned and couldn't do much other than stare at the knife, I had trouble following directions so the other kid had to physically restrain me (by which time I was coming to realize the seriousness of my situation and just complied), search me, take my wallet and jewelery and drugs. I then had as bad as a trip as I've ever with a touch of PTSD; since then I've also overreacted to provocation while tripping and started beating someone and had him on the ground and probably would have killed him if I had not been restrained by people from my group. Still yet also I've saved I've saved lives more than once by giving Narcan, once even CPR and even starting an IV. I've mediated a brewing potentially dangerous and very unbalanced territorial fight by competing organizations during a festival, not saving lives but a lot of unpleasantness, drama and retraction, lost revenue, and s on. So too have I driven, made love, etc. And every example of something not to do above.
But I'd never rely on myself to be able to do so again. That's part of the power and unpredictability of the drug.