Diabetes affects men and women differently when it comes to sex but all damage and issues come from the same reason. Unchecked diabetes damages blood vessels on a micro-vascular level and as such the vessels which are responsible for sex in general will be damaged too and cause all manner of unpleasant complications.
Men
The most common issue here is that of erectile dysfunction. Obviously getting and then maintaining a full erection requires a good level of blood flow, without that flow men could well struggle to get an erection in the first place and if they get one it may not last as long. Erectile dysfunction is considerably more common in type one diabetics than in the general population.
Another issue is around the area of ejaculation. When nerve damage occurs it may prevent a man from ejaculating properly, instead of semen being expelled it could end up in the bladder. This is called retrograde ejaculation and can cause considerable issues in those trying to conceive.
Women
Again, the issues in women are caused by damage to the blood vessels. Hardened blood vessels in the vaginal walls can make sex painful, lessen sex drive or cause vaginal dryness. Ladies might find it difficult, if not impossible to get around and they may also lose the ability to reach orgasm.