How can one have sex without fear of falling ill in between or after?
That's a great question. I don't think that you can really get rid of the fear until you have had successful sessions in the boudoir without a crises. It does take a while to find the best routine and regimen that works for you, and it all depends on your partner, and how you articulate your needs and limitations.
The truth of the matter is that you have to listen to what your body is telling you. Chances are, positions that have your knees and joints in odd angles are okay for a few minutes, but anything more than 4-5, and you will be hurting seriously after. You have to find knee friendly positions as your staples, like missionary for the ladies with sickle cell, and doggy style for the guys. You can switch it up and do other positions, but you shouldn't be trying to work through the Kama Sutra or folding your body up like a pretzel in the name of good sex.
Make sure you are well hydrated before and after engaging in any serious bedplay. Heck, if you take a break between acts, drink some water then too! You are losing water through perspiration and bodily fluids, so replenishing them as soon as possible is essential to preventing a crises.
You know those twinges you get in certain parts of your body when you are engaging in strenuous activity? Do not ignore those twinges during sex. That is your body's early warning signal telling you that whatever position you are in, or whatever you are doing is way too much for you! Switch it up, take a break, put your legs down and stretch out your limbs.
Ease into it. Sex is exercise, and when you are going to the gym, you always do stretches before you hit the treadmill. Stretching improves the blood flow to your muscles, prevents spasms, cramps, and heats up the body gradually thereby preventing lactic acid buildup in the muscles. Since during the plateau phase of sex (right before orgasm), your muscles automatically tense up, it's good to have them warm and supple when this occurs.
Oh, and chances are, with sickle cell, you aren't going to be a Sex Olympian. There will be no 2-3 hour bedroom sessions for you...in fact, you will be lucky if you make it to the 45 minute mark (this includes foreplay), without getting tired and fatigued. Remember, we are naturally anemic, which means that we are biologically at a slight disadvantage to those with normal hemoglobin count.
To overcome this limitation, I would suggest expanding your repertoire to include other things besides coitus. Think feathers, tantra, erotic massages, erotica, light touches, oral, BJs etc. If you get him (or her) riled up enough before penetration, chances are, your significant other won't even notice that it's been less than an hour for the whole event.
The hardest part for me was finding a partner that was patient, intelligent and loving enough to have sex with me without inducing a crises. It takes a special person to be able to understand the implications of SCD, and have multiple successful encounters in the bedroom without triggering a crises. Once you are able to do that, then you will have the comfort level to explore the outer realms of your desire, and still contain that SCD beast within.