This is a good question and tricky to get right / lots to consider. For me the following is true (and for reference my normal range is 4-8 that I achieve around 85-90% of the time).
Less than 4: we don't have lift off
4 - 4.5: not ideal but ok
4.5 - 6.5: all systems go, in the 5s is perfect. If can stay here then hours of fun possible.
6.5 - 7: not ideal but ok
7-9: would psychologically really rather not have erections as damage being caused to blood vessels and nerves.
Above 9: we don't have lift off (might still be psycholigical, but also physical as lower levels of nitric oxide being released that are needed for erection), but the thought of erect penis being damaged by high sugars is not appealing. Plus at these levels i'm feeling lethargic and brain fog, not what you want for sex.
So 4.5 - 6.5 is a very tight range.
For me one of the biggest frustrations of T1 in that this makes spontaneity very difficult. Achieving that stable range takes planning. A nice evening meal, then romatic evening... ok, but lets low carb the meal and wait 3 hours for the main action.
The advice to not have much short acting insulin on board is good (so at least c3+ hours) as that should stop any rapid drops.
Starting higher within the 4.5-6.5 range makes sense to allow for lowering of BS. So if i was at eg 4.8 before things began, i might take some short acting carbs while freshing up, enough to get into the 6s. But while I might start sports exercise higher eg up to 10, that is just not possible and certainly not desirable in terms of erections and long term erectile health.
In terms of when to test again, well I have a CGM, but maybe go on feeling? But if you unexpectedly lose / can't get erection then test then, might be you're low or high, both of which kill erections.
Communication with partner is very good idea, some won't be supportive but others will. Another good chance to weed out the bad ones.
