Part of the incredible work that researchers are doing all over the world, there was a study done to assess the tolerability of a gel that may prevent the transmission of HIV.
The full study protocol is here, it is a very long read, though of interest may be the participant information sheet on page 102: https://drive.google.com/open?id=1Ez_yMzonjWbU4f4U_SfE98koi5an7XAv
In the study, participants first had standard screening exams, including a rectal swabs
Before study days, the participants would donate semen, which was then mixed with radioactive labels so that its spread could later be seen on a scan.
On study days, the participants were examined, then given an enema. Then, they would have the gel squeezed into their rectum.
They had a cannula placed for regular blood sampling.
On half of the study days, the 'coital simulation' days, one hour after the gel had been squeezed into their rectum, the participants would then 'move a dildo in and out' to a set rythm, once per second, for 5 minutes.
The dildo had been drilled and a catheter placed through its length. At the end of the 5 minutes, a clinican would then inject their own semen sample, that had been radiolabelled, through the dildo - simulating an ejaculation.
The participant would then fuck themselves another 10 times with the dildo, before having CT scans to see how the 'ejaculation' had spread through their body.
May provide some ideas for your own play...