Our Gemara on Amud Aleph asks whether it is permitted to extinguish a lamp on Yom Tov in order to have marital relations.  What is the reason that it is considered important to extinguish the lamp before engaging in marital relations?  It is considered to be improper and immodest to engage in sexual intercourse where the private parts of the body can be seen. This is codified in Shulkhan Arukh EH 25:5 and OH 240. This prohibition naturally extends to having intercourse in the daytime and the various commentaries discuss situations where it may be permitted such as for shlom bayis, provided great care is taken to stay as much under covers as possible.

Some interesting sources not discussed in Shulkhan Arukh are as follows: 

Based on the plain reading of the Biblical text (Bereishis 26:8), Ohr Hachaim notes that apparently Issac and Rivkah had relations during the day.  In addition the Gemara (Niddah 17a) discusses people from the House of Munbaz (a political family) who worked long hours and therefore were unable to properly conduct marital relations at night. The Gemara praises them for choosing to engage in relations during the day, so that the mitzvah could be fulfilled properly.  

What does it mean that they were too tired to fulfill the mitzvah properly at night?  The Gemara says that their exhaustion would lead to a sense of contempt and disgust instead of attraction.  Rashi elaborates further and states that from the exhaustion he will not sufficiently desire her nor sufficiently appease her, leading to sexual intercourse “only for the sake of the mitzvah”.  This Rashi is worth noting, because he considers intercourse “only for the sake of the mitzvah” without genuine desire, to be abominable.

I must wonder, if the rabbis praised the house of Munbaz for subverting the modesty concerns of daytime in order to allow for healthy expression of desire, would this also apply to the many other modesty restrictions delineated in Shulkhan Arukh?  Perhaps the idea expressed by Ramah (EH 25:2) that all matters are technically permitted between husband and wife within certain bounds (see commentary Op. Cit. for details) is not just an afterthought but something to be taken seriously if these restrictions serve as barrier to desire.