It says in Shulchan Aruch Siman 240 Sif 11 that it's prohibited to have marital relations by day unless the house is dark ("Bayis Afel".) The Rema adds that a Talmid Chacham that covers himself with his talis (or seemingly a sheet or some sort of cover) is also permitted to have marital relations by day. The Mishneh Brurah in Sif-Katan 45 explains that since a Talmid Chacham is modest in his ways he won't come to look (at his wife in the light.) Therefore it's permitted to have relations if he covers himself well. However the the M"B ends off that this is only to be done in a case of tzrich gadol or if his yatzer is misgaver over him.
What does it say about this inyun in other seforim or poskim? Are people more makil then the Mishneh Brurah now a days or even less? Obviously it's hard to take a survey in this however perhaps what we see from other seforim or poskim (or any "shmu'os") would be a good way to tell.
The Shulchan Aruch speaks about a case where the entire house is dark and one is not relying on the heter of using a talis. Seemingly in that case there is more of a reason to be makil (even without a "tzrich gadol" -- great need. I later saw that in the Bar Hativ Siman 21 this is what he says from the Magen Avraham based on the Gemorah in Kasubos Daf 65.) It's interesting also that the Rema speaks about being matir for a Talmid Chacham (only the reason makes sense since he is anyway more modest in his ways) why is it that the Talmid Chacham has such a "yatzer" to have relations by day.