Hi Jimbo, though my situation was slightly different then yours, be it that this involved my wifes boyfriend, but similar in the sense it was a new relationship for them.
When he would come to visit, we would all typically share the large king size bed we have, and often times they would have sex, even with me in the bed with them, but I would not participate, and typically they would sleep close together through the rest of the night. Since he was able to visit on very rare occasion, about 4 times a year, I wanted them to have the opportunity to have alone time. So on the second or third night of his visit, I offered to sleep in the guest room so they could be by themselves. Throughout the evening, as we hung out, made dinner and had drinks, the more nervous I felt about them being alone when we decided to go to bed. Finally we decided to head to bed, we walked upstairs, and at the top of the stairs, kissed my wife goodnight and her boyfriend thanked me, before heading into separate rooms.
Just like you said, my heart raced knowing they were having sex in the other room, that they would sleep close and cuddling through the night. Finally I fell asleep and woke early the next morning and went downstairs, drank coffee and went online, killing time while they continued to sleep, exhausted from the night before. Awhile later, I could hear they had woken up, but did not get out of bed, I actually could hear them having sex for a bit, then I believe they fell back to sleep again. All of which made my heart race and felt a bit alone.
Finally, I have to admit, it seemed like forever they came downstairs and we all hung out, had coffee and I made breakfast for all of us.
Seeing the smile on my wifes face, having had the opportunity to spend time alone with her boyfriend actually made me feel really good. Her boyfriend was very appreciative also and once again thanked me for the time alone with her. I knew that after he left at the end of the week, it would be months before they would see each other, so I did make a point of it to give them as much alone time also. I think the feeling of compersion over powered any jealousy that could have been there, and the anticipation is so much worse then when something actually happens. But they are all normal feelings.
You should feel good about yourself that you give your girlfriend and her girlfriend some time together alone, and the appreciation you get in return from her makes it all worth it, and her happiness is even more payback.