I submit that it is. Or rather, I submit that custom software can be romantic, subject to its content. Moreover, I submit that custom software is more romantic than the equivalent non-custom item.
This seems obvious to me. Handmade is always more touching than purchased, all else being equal. A flowery store-bought card is romantic,** but not as romantic as identical sentiments composed by the bearer. Buying your boyfriend his favorite cookies is sweet; making his favorite cookies is sweeter. Giving someone a sweater from Macy's is nice; making them a sweater is nicer. That doesn't mean all gifts should be crafted by the giver; sentiment is only one of the purposes of a gift, and making something isn't the only way to be sentimental. Carefully selecting a store-bought gift is no less romantic, and often more so, than giving an unwanted or ineptly-manufactured item just because one has made it oneself.
Why should software be any different? Software is, if anything, more versatile in its romantic potential. One could write software for one's beloved with no purpose besides expressing one's feelings (for example, an animated graphic involving hearts, or etcetera); this is analogous to composing a poem. Or one could write useful software that would improve the recipient's life (and that presumably is not otherwise available); this is like knitting a blanket for someone who is always cold.
So I think custom software can certainly be romantic, and is potentially*** just as romantic as any other romantic gift.
* Disclaimer: None of this should be interpreted as sour grapes over the impending V-Day. If you have been here a while, you know that I always like V-Day.
** I don't buy the faux-purist bullshit that store-bought cards are inherently evil or stupid or devoid of meaning. The line of reasoning seems to be that anyone can buy a card, which is true, but not everyone does. The card was still chosen for what it says and its visual imagery, and for a lot of people it's much more accurate and poetic to let someone else verbalize their feelings. The argument against cards seems to be that people can choose a card that intentionally misrepresents their feelings, but of course people can lie in their own words as well.
*** I say potentially because level-of-romance depends on many specifics about the gift, including intangibles such as its intent, presentation, and nature of the relationship. Also, romance doesn't necessarily equate to usefulness or happiness; there is a certain very compelling breed of romance that is inherently doomed. Also mostly of all, I find it hard to compare levels of romance across time, situations, and people in any meaningful way. I have received jewelry in a romantic context, and I have received really unromantic jewelry. I have received romantic and unromantic flowers and romantic and unromantic chocolate. I have given books that I intended to be romantic, and flowers and chocolates that I intended not to be. One of the more romantic gifts I have been given or even heard of was a powerpoint presentation.